feat(fp): WP-25 — server-rendered letter HTML preview

Adds LetterHtml.Render, a pure composer mirroring the FE letter canvas'
class vocabulary, behind two ExcludeFromDescription()'d endpoints
(GET /brief/preview, GET /admin/org-template/{subOrgId}/preview).
Auto-resolvable placeholders pull from seed/case data; unresolved
manual ones render as "[NOG IN TE VULLEN: label]". A sent brief
archives its composed HTML (BriefEntity.ArchivedHtml) so a later
org-template republish never changes it. FE gets a hand-written fetch
adapter (text/html, not JSON) and a "Voorbeeld" button that opens the
preview in a new tab.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/src/BigRegister.Api/Data/BriefStore.cs

@@ -1,5 +1,6 @@
 using BigRegister.Api.Contracts;
 using BigRegister.Domain.Authorization;
+using BigRegister.Domain.Letters;
 using Microsoft.EntityFrameworkCore;
 
 namespace BigRegister.Api.Data;
@@ -26,6 +27,9 @@ public sealed class BriefEntity
   public string SubOrgId { get; set; } = OrgTemplateSeed.Registers;
   /// Pinned at send: sent letters are immutable, a republish never re-themes them.
   public int? SentOrgTemplateVersion { get; set; }
+  /// The composed HTML archived at send (WP-25) — from here on the preview endpoint
+  /// serves this verbatim, so a later org-template republish never re-renders it.
+  public string? ArchivedHtml { get; set; }
 
   public BriefDto ToDto() => new(BriefId, Beroep, TemplateId, Placeholders, Sections, Status, DrafterId);
 }
@@ -110,6 +114,10 @@ public static class BriefStore
       // Pin the org-template version the letter was sent with (WP-23): from here on
       // its appearance is frozen — republishing the template touches unsent briefs only.
       e.SentOrgTemplateVersion = OrgTemplateStore.PublishedVersionOf(e.SubOrgId);
+      // Archive the composed HTML at this exact instant (WP-25): the preview endpoint
+      // serves this verbatim once sent, so a later republish never re-renders it.
+      var template = OrgTemplateStore.TemplateForBrief(e.SubOrgId, e.SentOrgTemplateVersion);
+      e.ArchivedHtml = LetterHtml.Render(e, template, at, watermark: false);
       db.SaveChanges();
       return (Outcome.Ok, e);
     }

backend/src/BigRegister.Api/Data/Migrations/20260705101743_ArchivedHtml.Designer.cs

@@ -0,0 +1,226 @@
+// <auto-generated />
+using System;
+using BigRegister.Api.Data;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
+
+#nullable disable
+
+namespace BigRegister.Api.Data.Migrations
+{
+    [DbContext(typeof(AppDbContext))]
+    [Migration("20260705101743_ArchivedHtml")]
+    partial class ArchivedHtml
+    {
+        /// <inheritdoc />
+        protected override void BuildTargetModel(ModelBuilder modelBuilder)
+        {
+#pragma warning disable 612, 618
+            modelBuilder.HasAnnotation("ProductVersion", "10.0.9");
+
+            modelBuilder.Entity("BigRegister.Api.Data.Aanvraag", b =>
+                {
+                    b.Property<string>("Id")
+                        .HasColumnType("TEXT");
+
+                    b.Property<bool>("AutoApprovable")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<DateTimeOffset>("CreatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("DocumentIds")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Draft")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Owner")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Reden")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Referentie")
+                        .HasColumnType("TEXT");
+
+                    b.Property<int>("StepCount")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<int>("StepIndex")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<bool>("Submitted")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<DateTimeOffset?>("SubmittedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Type")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("UpdatedAt")
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.ToTable("Applications");
+                });
+
+            modelBuilder.Entity("BigRegister.Api.Data.AuditEntry", b =>
+                {
+                    b.Property<long>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("INTEGER");
+
+                    b.Property<string>("Action")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Actor")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("At")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("CategoryId")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("DocumentId")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.ToTable("AuditEntries");
+                });
+
+            modelBuilder.Entity("BigRegister.Api.Data.BriefEntity", b =>
+                {
+                    b.Property<string>("BriefId")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("ArchivedHtml")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Beroep")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("DrafterId")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Owner")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Placeholders")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Sections")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<int?>("SentOrgTemplateVersion")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<string>("Status")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("SubOrgId")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("TemplateId")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("BriefId");
+
+                    b.HasIndex("Owner")
+                        .IsUnique();
+
+                    b.ToTable("Briefs");
+                });
+
+            modelBuilder.Entity("BigRegister.Api.Data.OrgTemplateEntity", b =>
+                {
+                    b.Property<string>("SubOrgId")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Draft")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("History")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<int>("PublishedVersion")
+                        .HasColumnType("INTEGER");
+
+                    b.HasKey("SubOrgId");
+
+                    b.ToTable("OrgTemplates");
+                });
+
+            modelBuilder.Entity("BigRegister.Api.Data.StoredDocument", b =>
+                {
+                    b.Property<string>("DocumentId")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("CategoryId")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<byte[]>("Content")
+                        .IsRequired()
+                        .HasColumnType("BLOB");
+
+                    b.Property<string>("ContentType")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("FileName")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<bool>("Linked")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<string>("LocalId")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Owner")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<long>("SizeBytes")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<DateTimeOffset>("UploadedAt")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("WizardId")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("DocumentId");
+
+                    b.ToTable("Documents");
+                });
+#pragma warning restore 612, 618
+        }
+    }
+}

backend/src/BigRegister.Api/Data/Migrations/20260705101743_ArchivedHtml.cs

@@ -0,0 +1,28 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace BigRegister.Api.Data.Migrations
+{
+  /// <inheritdoc />
+  public partial class ArchivedHtml : Migration
+  {
+    /// <inheritdoc />
+    protected override void Up(MigrationBuilder migrationBuilder)
+    {
+      migrationBuilder.AddColumn<string>(
+          name: "ArchivedHtml",
+          table: "Briefs",
+          type: "TEXT",
+          nullable: true);
+    }
+
+    /// <inheritdoc />
+    protected override void Down(MigrationBuilder migrationBuilder)
+    {
+      migrationBuilder.DropColumn(
+          name: "ArchivedHtml",
+          table: "Briefs");
+    }
+  }
+}

backend/src/BigRegister.Api/Data/Migrations/AppDbContextModelSnapshot.cs

@@ -104,6 +104,9 @@ namespace BigRegister.Api.Data.Migrations
                     b.Property<string>("BriefId")
                         .HasColumnType("TEXT");
 
+                    b.Property<string>("ArchivedHtml")
+                        .HasColumnType("TEXT");
+
                     b.Property<string>("Beroep")
                         .IsRequired()
                         .HasColumnType("TEXT");

backend/src/BigRegister.Api/Domain/Letters/LetterHtml.cs

@@ -0,0 +1,191 @@
+using System.Globalization;
+using System.Text;
+using BigRegister.Api.Contracts;
+using BigRegister.Api.Data;
+
+namespace BigRegister.Domain.Letters;
+
+/// <summary>
+/// Server-rendered letter HTML (WP-25) — the archived, "what is sent" artifact.
+/// Mirrors the FE letter canvas' class vocabulary exactly (<c>public/letter.css</c>,
+/// the FE⇄BE contract; LetterHtmlTests' class-parity test is the fence against drift).
+///
+/// Unlike the canvas, placeholders resolve to real text rather than a live editor
+/// widget: an auto-resolvable key pulls from seed/case data (there is no per-brief
+/// resolved value stored anywhere else in the domain — see BriefEntity's own "does
+/// not interpret it" posture), an unresolved manual key renders literally as
+/// "[NOG IN TE VULLEN: label]" (PRD Brief v2 §8) — the preview works despite this,
+/// only send blocks on it (FE-authoritative linting).
+///
+/// ponytail: HTML today, a headless-Chromium PDF render slots in behind this same
+/// route if the POC ever needs real PDF bytes — see the preview endpoints.
+/// </summary>
+public static class LetterHtml
+{
+  private static readonly string Css = File.ReadAllText(FindLetterCss());
+
+  public static string Render(BriefEntity brief, OrgTemplateDto template, string at, bool watermark)
+  {
+    var defs = brief.Placeholders.ToDictionary(p => p.Key);
+    var sb = new StringBuilder();
+
+    sb.Append("<!doctype html><html lang=\"nl\"><head><meta charset=\"utf-8\">");
+    sb.Append("<title>").Append(Enc(brief.BriefId)).Append("</title>");
+    sb.Append("<style>").Append(Css).Append(ExtraCss).Append("</style>");
+    sb.Append("</head><body>");
+    sb.Append("<div class=\"letter\" style=\"").Append(MarginStyle(template.Margins)).Append("\">");
+
+    // --- letterhead ---
+    sb.Append("<div class=\"letter__letterhead\">");
+    if (template.LogoDocumentId is { } logoId && DocumentStore.Get(logoId) is { } logo)
+    {
+      sb.Append("<img class=\"org-logo\" alt=\"\" src=\"data:").Append(logo.ContentType)
+          .Append(";base64,").Append(Convert.ToBase64String(logo.Content)).Append("\">");
+    }
+    sb.Append("<p class=\"org-wordmark\">").Append(Enc(template.OrgName)).Append("</p>");
+    sb.Append("<address class=\"return-address\">").Append(EncLines(template.ReturnAddress)).Append("</address>");
+    sb.Append("<address class=\"address-window\">").Append(EncLines(RecipientPlaceholder)).Append("</address>");
+    sb.Append("<dl class=\"reference\">");
+    sb.Append("<div><dt>Ons kenmerk</dt><dd>").Append(Enc(brief.BriefId)).Append("</dd></div>");
+    sb.Append("<div><dt>Datum</dt><dd>").Append(Enc(FormatDatumNl(at))).Append("</dd></div>");
+    sb.Append("</dl></div>");
+
+    // --- body: the case-type template's sections ---
+    sb.Append("<div class=\"letter__body\">");
+    foreach (var section in brief.Sections)
+    {
+      sb.Append("<section><h3>").Append(Enc(section.Title)).Append("</h3>");
+      foreach (var block in section.Blocks)
+        RenderParagraphs(sb, block.Content.Paragraphs, defs);
+      sb.Append("</section>");
+    }
+    sb.Append("</div>");
+
+    // --- signature ---
+    sb.Append("<div class=\"letter__signature\">");
+    sb.Append("<p>").Append(Enc(template.SignatureClosing)).Append("</p>");
+    sb.Append("<p class=\"signature-name\">").Append(Enc(template.SignatureName)).Append("</p>");
+    sb.Append("<p>").Append(Enc(template.SignatureRole)).Append("</p>");
+    sb.Append("</div>");
+
+    // --- footer ---
+    sb.Append("<div class=\"letter__footer\">");
+    sb.Append("<div class=\"footer-contact\">").Append(EncLines(template.FooterContact)).Append("</div>");
+    sb.Append("<div class=\"footer-legal\">").Append(Enc(template.FooterLegal)).Append("</div>");
+    sb.Append("</div>");
+
+    if (watermark) sb.Append("<div class=\"preview-watermark\" aria-hidden=\"true\">VOORBEELD</div>");
+
+    sb.Append("</div></body></html>");
+    return sb.ToString();
+  }
+
+  /// Exposed so LetterHtmlTests can assert every `letter`-prefixed class this
+  /// renderer emits also exists in the shared contract file — the fence against drift.
+  public static string StyleSheet => Css;
+
+  // No recipient address is tracked anywhere in this POC's brief domain (BRP lookup
+  // is out of scope here) — the canvas shows the same static placeholder text.
+  private const string RecipientPlaceholder = "Adres van de geadresseerde\n(wordt ingevuld bij verzending)";
+
+  private static void RenderParagraphs(
+      StringBuilder sb, IReadOnlyList<ParagraphDto> paragraphs, IReadOnlyDictionary<string, PlaceholderDefDto> defs)
+  {
+    string? openList = null;
+    foreach (var para in paragraphs)
+    {
+      if (para.List != openList)
+      {
+        if (openList is not null) sb.Append(openList == "bullet" ? "</ul>" : "</ol>");
+        if (para.List is not null) sb.Append(para.List == "bullet" ? "<ul>" : "<ol>");
+        openList = para.List;
+      }
+      sb.Append(openList is null ? "<p>" : "<li>");
+      foreach (var node in para.Nodes) RenderNode(sb, node, defs);
+      sb.Append(openList is null ? "</p>" : "</li>");
+    }
+    if (openList is not null) sb.Append(openList == "bullet" ? "</ul>" : "</ol>");
+  }
+
+  private static void RenderNode(
+      StringBuilder sb, RichTextNodeDto node, IReadOnlyDictionary<string, PlaceholderDefDto> defs)
+  {
+    switch (node.Type)
+    {
+      case "text":
+        sb.Append(Enc(node.Text ?? ""));
+        break;
+      case "lineBreak":
+        sb.Append("<br>");
+        break;
+      case "placeholder":
+        var key = node.Key ?? "";
+        var def = defs.GetValueOrDefault(key);
+        var label = def?.Label ?? key;
+        sb.Append(def is { AutoResolvable: true } ? Enc(ResolveAuto(key, label)) : Enc($"[NOG IN TE VULLEN: {label}]"));
+        break;
+    }
+  }
+
+  // The only place a placeholder key gets a real value: seed/case data for the
+  // single demo applicant (SeedData.Registration — no per-brief resolved value is
+  // ever stored, see the class doc above). Falls back to the label itself for any
+  // other auto-resolvable key, mirroring the FE canvas' own `sampleFor` fallback.
+  private static string ResolveAuto(string key, string label) => key switch
+  {
+    "naam_zorgverlener" => SeedData.Registration.Naam,
+    "big_nummer" => SeedData.Registration.BigNummer,
+    "datum" => FormatDatumNl(DateTimeOffset.UtcNow.ToString("o")),
+    _ => label,
+  };
+
+  private static readonly CultureInfo Nl = CultureInfo.GetCultureInfo("nl-NL");
+  private static string FormatDatumNl(string at) => DateTimeOffset.Parse(at).ToString("d MMMM yyyy", Nl);
+
+  private static string MarginStyle(MarginsDto m) =>
+      $"--letter-margin-top:{m.TopMm}mm;--letter-margin-right:{m.RightMm}mm;" +
+      $"--letter-margin-bottom:{m.BottomMm}mm;--letter-margin-left:{m.LeftMm}mm;";
+
+  private static string Enc(string s) => System.Net.WebUtility.HtmlEncode(s);
+  private static string EncLines(string s) => Enc(s).Replace("\n", "<br>");
+
+  // Walks up from the running assembly's own directory (NOT the process cwd, which
+  // varies by how `dotnet run`/docker/tests invoke it — see docs/backlog/WP-25) until
+  // it finds `public/letter.css`. docker-compose.yml bind-mounts `./public` under the
+  // api container's `/src` for exactly this walk to resolve there too.
+  private static string FindLetterCss()
+  {
+    for (var dir = new DirectoryInfo(AppContext.BaseDirectory); dir is not null; dir = dir.Parent)
+    {
+      var candidate = Path.Combine(dir.FullName, "public", "letter.css");
+      if (File.Exists(candidate)) return candidate;
+    }
+    throw new FileNotFoundException(
+        $"public/letter.css not found by walking up from {AppContext.BaseDirectory} " +
+        "— check the docker bind mount or build output location.");
+  }
+
+  // Backend-only concerns absent from the FE canvas (no live preview toggle for
+  // either): kept out of the shared contract file, not "letter"-prefixed so the
+  // class-parity test's scope doesn't need to widen for them.
+  private const string ExtraCss = """
+      .preview-watermark {
+        position: fixed;
+        inset: 0;
+        display: flex;
+        align-items: center;
+        justify-content: center;
+        font-size: 72pt;
+        font-weight: 700;
+        color: rgb(200 30 30 / 0.18);
+        transform: rotate(-30deg);
+        pointer-events: none;
+        z-index: 3;
+      }
+      .org-logo {
+        display: block;
+        max-height: 18mm;
+        margin-block-end: 4mm;
+      }
+      """;
+}

backend/src/BigRegister.Api/Program.cs

@@ -346,6 +346,31 @@ api.MapPost("/brief/send", (HttpContext ctx) =>
 .Produces<BriefViewDto>()
 .ProducesProblem(StatusCodes.Status409Conflict);
 
+// Server-rendered HTML preview (WP-25): "what you compose is what is sent" — the
+// same LetterHtml.Render a sent brief archived. Hand-written on the FE (fetch →
+// blob → new tab), so excluded from the OpenAPI doc, same seam as uploads. Sent
+// letters serve their frozen archive; anything else renders live with a watermark.
+api.MapGet("/brief/preview", (HttpContext ctx) =>
+{
+  var e = BriefStore.GetOrCreate(DocumentStore.DemoOwner);
+  if (e.Status.Tag == "sent" && e.ArchivedHtml is { } archived)
+    return Results.Content(archived, "text/html");
+  var template = OrgTemplateStore.TemplateForBrief(e.SubOrgId, null);
+  return Results.Content(LetterHtml.Render(e, template, Now(), watermark: true), "text/html");
+})
+.ExcludeFromDescription();
+
+// Proefbrief: the admin's unpublished draft template rendered over a fixture
+// brief, so the appearance can be checked before publishing touches real letters.
+api.MapGet("/admin/org-template/{subOrgId}/preview", (string subOrgId, HttpContext ctx) => OrgAdmin(ctx, () =>
+{
+  var view = OrgTemplateStore.AdminView(subOrgId);
+  if (view is null) return Results.NotFound();
+  var fixture = BriefSeed.NewBrief("proefbrief");
+  return Results.Content(LetterHtml.Render(fixture, view.Draft, Now(), watermark: true), "text/html");
+}))
+.ExcludeFromDescription();
+
 api.MapPost("/brief/reset", (HttpContext ctx) =>
 {
   // Demo "start over": recreate a fresh draft. No guards — showcase affordance only.

backend/tests/BigRegister.Tests/BigRegister.Tests.csproj

@@ -23,4 +23,10 @@
     <ProjectReference Include="..\..\src\BigRegister.Api\BigRegister.Api.csproj" />
   </ItemGroup>
 
+  <ItemGroup>
+    <Content Include="LetterHtml.golden.html">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
+  </ItemGroup>
+
 </Project>

backend/tests/BigRegister.Tests/LetterHtml.golden.html

@@ -0,0 +1,215 @@
+<!doctype html><html lang="nl"><head><meta charset="utf-8"><title>golden-brief-1</title><style>/* letter.css — the FE⇄BE letter-rendering CONTRACT (WP-24/WP-25).
+ *
+ * One stylesheet, two consumers: the FE letter canvas loads it via <link>
+ * (index.html + Storybook preview-head), the backend HTML renderer (WP-25)
+ * inlines this same file. Its class-parity test is the fence against drift.
+ *
+ * Class vocabulary: .letter, .letter__letterhead, .letter__body,
+ * .letter__signature, .letter__footer, .letter__page-break.
+ * Margins arrive as --letter-margin-* custom props (mm, from the OrgTemplate);
+ * the defaults below match the seed template.
+ *
+ * Deliberately self-contained: no --rhc- or --bs- tokens — the backend renderer
+ * has no token bridge. Geometry follows the sample voorbeeldbrief-inschrijving
+ * (A4, return address above the envelope window, reference block, footer rule).
+ */
+
+.letter {
+  --letter-margin-top: 25mm;
+  --letter-margin-right: 25mm;
+  --letter-margin-bottom: 25mm;
+  --letter-margin-left: 25mm;
+
+  position: relative;
+  box-sizing: border-box;
+  width: 210mm; /* A4 */
+  max-width: 100%;
+  min-height: 297mm;
+  margin-inline: auto;
+  padding: var(--letter-margin-top) var(--letter-margin-right) var(--letter-margin-bottom)
+    var(--letter-margin-left);
+  background: #fff;
+  color: #1a1a1a;
+  /* Letter typography is the letter's, not the portal UI's. Licensed RO/Rijks
+     fonts are not shipped; system stack mirrors the app-wide decision (ADR-0003). */
+  font-family:
+    system-ui,
+    -apple-system,
+    'Segoe UI',
+    Roboto,
+    sans-serif;
+  font-size: 10.5pt;
+  line-height: 1.5;
+  display: flex;
+  flex-direction: column;
+}
+
+.letter p {
+  margin: 0 0 0.75em;
+}
+
+.letter ul,
+.letter ol {
+  margin: 0 0 0.75em;
+  padding-inline-start: 1.4em;
+}
+
+/* --- Letterhead: wordmark, return address above the envelope window, reference block --- */
+
+.letter__letterhead {
+  margin-block-end: 12mm;
+}
+
+.letter__letterhead .org-wordmark {
+  font-size: 13pt;
+  font-weight: 700;
+  margin: 0 0 10mm;
+}
+
+.letter__letterhead .return-address {
+  font-style: normal;
+  font-size: 7.5pt;
+  white-space: pre-line;
+  color: #555;
+  margin-block-end: 2mm;
+}
+
+/* Envelope-window position (~C5 venstercouvert): recipient block. */
+.letter__letterhead .address-window {
+  min-height: 22mm;
+  font-style: normal;
+  white-space: pre-line;
+  margin-block-end: 8mm;
+}
+
+.letter__letterhead .reference {
+  display: flex;
+  gap: 10mm;
+  font-size: 8.5pt;
+  color: #333;
+}
+
+.letter__letterhead .reference dt {
+  font-weight: 700;
+  margin: 0;
+}
+
+.letter__letterhead .reference dd {
+  margin: 0;
+}
+
+/* --- Body: the case-type template's sections --- */
+
+.letter__body {
+  flex: 1;
+  /* Above the absolutely-positioned page-break marks: the dashed line stays visible
+     in the gaps but never draws THROUGH opaque content (editors, pickers). */
+  position: relative;
+  z-index: 1;
+}
+
+.letter__body h3 {
+  font-size: inherit;
+  font-weight: 700;
+  margin: 0 0 0.25em;
+}
+
+.letter__body section {
+  margin-block-end: 1.5em;
+}
+
+/* --- Signature --- */
+
+.letter__signature {
+  margin-block-start: 10mm;
+}
+
+.letter__signature p {
+  margin: 0;
+}
+
+.letter__signature .signature-name {
+  margin-block-start: 3em; /* room for the (not-shipped) handwritten signature */
+  font-weight: 700;
+}
+
+/* --- Footer: contact + legal, above a rule --- */
+
+.letter__footer {
+  margin-block-start: 10mm;
+  padding-block-start: 3mm;
+  border-block-start: 0.5pt solid #999;
+  font-size: 7.5pt;
+  color: #555;
+  display: flex;
+  justify-content: space-between;
+  gap: 10mm;
+}
+
+.letter__footer .footer-contact {
+  white-space: pre-line;
+}
+
+.letter__footer .footer-legal {
+  text-align: end;
+}
+
+/* --- Approximate page-break indicator (screen only; PRD §2b honesty rule) ---
+   Positioned per A4-interval by the canvas; the print preview is authoritative. */
+
+.letter__page-break {
+  position: absolute;
+  inset-inline: 0;
+  border-block-start: 1px dashed #b36200;
+  pointer-events: none;
+}
+
+.letter__page-break > span {
+  position: absolute;
+  inset-inline-end: 2mm;
+  inset-block-start: 0;
+  transform: translateY(-50%);
+  /* Above .letter__body: the honesty caption stays legible even over content. */
+  z-index: 2;
+  font-size: 7pt;
+  color: #b36200;
+  background: #fff;
+  padding-inline: 1mm;
+  white-space: nowrap;
+}
+
+/* --- Print: real pages, no indicator chrome --- */
+
+@page {
+  size: A4;
+  margin: 0;
+}
+
+@media print {
+  .letter {
+    width: auto;
+    min-height: 0;
+  }
+
+  .letter__page-break {
+    display: none;
+  }
+}
+.preview-watermark {
+  position: fixed;
+  inset: 0;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  font-size: 72pt;
+  font-weight: 700;
+  color: rgb(200 30 30 / 0.18);
+  transform: rotate(-30deg);
+  pointer-events: none;
+  z-index: 3;
+}
+.org-logo {
+  display: block;
+  max-height: 18mm;
+  margin-block-end: 4mm;
+}</style></head><body><div class="letter" style="--letter-margin-top:25mm;--letter-margin-right:20mm;--letter-margin-bottom:20mm;--letter-margin-left:25mm;"><div class="letter__letterhead"><p class="org-wordmark">BIG-register</p><address class="return-address">Retouradres: Postbus 00000, 2500 AA Den Haag</address><address class="address-window">Adres van de geadresseerde<br>(wordt ingevuld bij verzending)</address><dl class="reference"><div><dt>Ons kenmerk</dt><dd>golden-brief-1</dd></div><div><dt>Datum</dt><dd>5 juli 2026</dd></div></dl></div><div class="letter__body"><section><h3>Aanhef</h3><p>Geachte heer/mevrouw Dr. A. (Anna) de Vries,</p></section><section><h3>Kern van het besluit</h3><p>Op </p><ul><li>Eerste punt: [NOG IN TE VULLEN: Reden besluit]</li><li>Tweede punt</li></ul></section><section><h3>Slot</h3><p>Met vriendelijke groet,</p></section></div><div class="letter__signature"><p>Met vriendelijke groet,</p><p class="signature-name">A. de Vries</p><p>Hoofd Registratie, BIG-register</p></div><div class="letter__footer"><div class="footer-contact">BIG-register &#183; Postbus 00000, 2500 AA Den Haag &#183; 070 000 00 00 &#183; info@voorbeeld.example</div><div class="footer-legal">Dit is een gegenereerd voorbeelddocument uit de register-reference PoC. Alle gegevens zijn fictief.</div></div><div class="preview-watermark" aria-hidden="true">VOORBEELD</div></div></body></html>

backend/tests/BigRegister.Tests/LetterHtmlTests.cs

@@ -0,0 +1,102 @@
+using System.Text.RegularExpressions;
+using BigRegister.Api.Contracts;
+using BigRegister.Api.Data;
+using BigRegister.Domain.Letters;
+
+namespace BigRegister.Tests;
+
+/// <summary>
+/// WP-25's fence against drift between the backend renderer and the FE letter
+/// canvas: a golden-file snapshot of a fixed brief + template, and a class-parity
+/// check that every `letter`-prefixed class the renderer emits exists in the
+/// shared `public/letter.css` contract. Neither test launches a browser.
+/// </summary>
+public class LetterHtmlTests
+{
+  private static BriefEntity FixtureBrief() => new()
+  {
+    BriefId = "golden-brief-1",
+    Owner = "golden",
+    Beroep = "arts",
+    TemplateId = "besluit-arts",
+    DrafterId = BriefStore.DrafterId,
+    Placeholders = new[]
+    {
+      new PlaceholderDefDto("naam_zorgverlener", "Naam zorgverlener", true),
+      new PlaceholderDefDto("datum", "Datum", true),
+      new PlaceholderDefDto("reden_besluit", "Reden besluit", false),
+    },
+    Sections = new()
+    {
+      new("aanhef", "Aanhef", true, new List<LetterBlockDto>
+      {
+        new("freeText", "aanhef-1", new RichTextBlockDto(new[]
+        {
+          new ParagraphDto(new[]
+          {
+            new RichTextNodeDto("text", Text: "Geachte heer/mevrouw "),
+            new RichTextNodeDto("placeholder", Key: "naam_zorgverlener"),
+            new RichTextNodeDto("text", Text: ","),
+          }),
+        })),
+      }, Locked: true),
+      new("kern", "Kern van het besluit", true, new List<LetterBlockDto>
+      {
+        new("freeText", "kern-1", new RichTextBlockDto(new[]
+        {
+          new ParagraphDto(new[] { new RichTextNodeDto("text", Text: "Op ") }),
+          new ParagraphDto(new RichTextNodeDto[]
+          {
+            new("text", Text: "Eerste punt: "),
+            new("placeholder", Key: "reden_besluit"),
+          }, List: "bullet"),
+          new ParagraphDto(new RichTextNodeDto[]
+          {
+            new("text", Text: "Tweede punt"),
+          }, List: "bullet"),
+        })),
+      }),
+      new("slot", "Slot", false, new List<LetterBlockDto>
+      {
+        new("freeText", "slot-1", new RichTextBlockDto(new[]
+        {
+          new ParagraphDto(new[] { new RichTextNodeDto("text", Text: "Met vriendelijke groet,") }),
+        })),
+      }, Locked: true),
+    },
+    Status = new BriefStatusDto("draft"),
+  };
+
+  private static readonly OrgTemplateDto Template = new(
+      "cibg-registers", "BIG-register",
+      "Retouradres: Postbus 00000, 2500 AA Den Haag",
+      LogoDocumentId: null,
+      "BIG-register · Postbus 00000, 2500 AA Den Haag · 070 000 00 00 · info@voorbeeld.example",
+      "Dit is een gegenereerd voorbeelddocument uit de register-reference PoC. Alle gegevens zijn fictief.",
+      "A. de Vries", "Hoofd Registratie, BIG-register", "Met vriendelijke groet,",
+      new MarginsDto(25, 20, 20, 25), Version: 1);
+
+  private const string At = "2026-07-05T12:00:00.0000000+00:00";
+
+  private static readonly string GoldenPath = Path.Combine(AppContext.BaseDirectory, "LetterHtml.golden.html");
+
+  [Fact]
+  public void Render_matches_the_golden_file()
+  {
+    var html = LetterHtml.Render(FixtureBrief(), Template, At, watermark: true);
+    var golden = File.ReadAllText(GoldenPath);
+    Assert.Equal(golden, html);
+  }
+
+  [Fact]
+  public void Every_letter_prefixed_class_exists_in_letter_css()
+  {
+    var html = LetterHtml.Render(FixtureBrief(), Template, At, watermark: true);
+    var classes = Regex.Matches(html, "class=\"([^\"]+)\"")
+        .SelectMany(m => m.Groups[1].Value.Split(' '))
+        .Where(c => c.StartsWith("letter"))
+        .Distinct();
+    Assert.NotEmpty(classes);
+    Assert.All(classes, c => Assert.Contains($".{c}", LetterHtml.StyleSheet));
+  }
+}

backend/tests/BigRegister.Tests/PreviewEndpointTests.cs

@@ -0,0 +1,96 @@
+using System.Net;
+using System.Net.Http.Json;
+using BigRegister.Api.Contracts;
+using BigRegister.Api.Data;
+using Microsoft.AspNetCore.Mvc.Testing;
+
+namespace BigRegister.Tests;
+
+/// <summary>
+/// WP-25: the two HTML preview endpoints. Both are excluded from the OpenAPI doc
+/// (see the drift check in the API-client generation step) — these tests hit them
+/// as plain HTTP, the same way the hand-written FE fetch does.
+/// </summary>
+public class PreviewEndpointTests(TestWebApplicationFactory factory) : IClassFixture<TestWebApplicationFactory>
+{
+  private const string Registers = OrgTemplateSeed.Registers;
+  private readonly HttpClient _client = factory.CreateClient();
+
+  private static LetterBlockDto FreeText(string id) =>
+      new("freeText", id, new RichTextBlockDto(new[] { new ParagraphDto(new[] { new RichTextNodeDto("text", Text: "inhoud") }) }));
+
+  private static SaveBriefRequest FilledFrom(BriefDto brief)
+  {
+    var i = 0;
+    var sections = brief.Sections
+        .Select(s => new LetterSectionDto(s.SectionKey, s.Title, s.Required, s.Required ? new[] { FreeText($"local-{++i}") } : s.Blocks))
+        .ToList();
+    return new SaveBriefRequest(sections);
+  }
+
+  private static void ResetStores()
+  {
+    BriefStore.Reset();
+    OrgTemplateStore.Reset();
+  }
+
+  private HttpRequestMessage Req(HttpMethod method, string path, string? role = null) =>
+      role is null ? new HttpRequestMessage(method, path) : new HttpRequestMessage(method, path) { Headers = { { "X-Role", role } } };
+
+  [Fact]
+  public async Task Preview_of_an_unsent_brief_renders_live_with_a_watermark()
+  {
+    ResetStores();
+    await _client.GetAsync("/api/v1/brief"); // GetOrCreate the demo draft
+
+    var res = await _client.GetAsync("/api/v1/brief/preview");
+    res.EnsureSuccessStatusCode();
+    Assert.Equal("text/html", res.Content.Headers.ContentType?.MediaType);
+    var html = await res.Content.ReadAsStringAsync();
+    Assert.Contains("<div class=\"preview-watermark\"", html);
+  }
+
+  [Fact]
+  public async Task Preview_of_a_sent_brief_serves_the_archive_unchanged_after_a_republish()
+  {
+    ResetStores();
+    var brief = (await _client.GetFromJsonAsync<BriefViewDto>("/api/v1/brief"))!.Brief;
+    await _client.PutAsJsonAsync("/api/v1/brief", FilledFrom(brief));
+    await _client.SendAsync(Req(HttpMethod.Post, "/api/v1/brief/submit"));
+    await _client.SendAsync(Req(HttpMethod.Post, "/api/v1/brief/approve", role: "approver"));
+    await _client.SendAsync(Req(HttpMethod.Post, "/api/v1/brief/send"));
+
+    var sentHtml = await (await _client.GetAsync("/api/v1/brief/preview")).Content.ReadAsStringAsync();
+    Assert.DoesNotContain("<div class=\"preview-watermark\"", sentHtml);
+    Assert.Contains("BIG-register", sentHtml);
+
+    // Republish the org template under a new name.
+    var adminView = await _client.SendAsync(Req(HttpMethod.Get, $"/api/v1/admin/org-template/{Registers}", role: "admin"));
+    var draft = (await adminView.Content.ReadFromJsonAsync<OrgTemplateAdminViewDto>())!.Draft;
+    await _client.SendAsync(new HttpRequestMessage(HttpMethod.Put, $"/api/v1/admin/org-template/{Registers}")
+    {
+      Headers = { { "X-Role", "admin" } },
+      Content = JsonContent.Create(new SaveOrgTemplateRequest(draft with { OrgName = "Hertitelde organisatie" })),
+    });
+    await _client.SendAsync(Req(HttpMethod.Post, $"/api/v1/admin/org-template/{Registers}/publish", role: "admin"));
+
+    // The sent brief's preview is unchanged — still the archived rendering.
+    var afterRepublish = await (await _client.GetAsync("/api/v1/brief/preview")).Content.ReadAsStringAsync();
+    Assert.Equal(sentHtml, afterRepublish);
+    Assert.DoesNotContain("Hertitelde organisatie", afterRepublish);
+  }
+
+  [Fact]
+  public async Task Proefbrief_is_admin_only_and_renders_the_draft_template()
+  {
+    ResetStores();
+    Assert.Equal(HttpStatusCode.Forbidden,
+        (await _client.GetAsync($"/api/v1/admin/org-template/{Registers}/preview")).StatusCode);
+
+    var res = await _client.SendAsync(Req(HttpMethod.Get, $"/api/v1/admin/org-template/{Registers}/preview", role: "admin"));
+    res.EnsureSuccessStatusCode();
+    var html = await res.Content.ReadAsStringAsync();
+    Assert.Contains("BIG-register", html);
+    Assert.Contains("<div class=\"preview-watermark\"", html);
+  }
+}

docker-compose.yml

@@ -16,6 +16,11 @@ services:
       - ./backend:/src:z
       - api-bin:/src/src/BigRegister.Api/bin
       - api-obj:/src/src/BigRegister.Api/obj
+      # WP-25: LetterHtml.Render inlines public/letter.css (the FE⇄BE letter
+      # contract, repo-root sibling of backend/) — mounted here so the same
+      # walk-up-from-the-assembly lookup that works for `dotnet run`/tests also
+      # resolves inside this container, whose bind mount otherwise only sees backend/.
+      - ./public:/src/public:z
     ports:
       - '5000:5000'
 

docs/backlog/WP-25-letter-preview-html.md

@@ -1,6 +1,6 @@
 # WP-25 — Server-rendered letter preview (HTML; PDF seam deferred)
 
-Status: todo
+Status: done
 Phase: 6 — Brief v2 (edit-on-the-letter, org templates, server-rendered preview)
 
 ## Why
@@ -63,13 +63,13 @@ same composition is archived with the brief, making sent letters immutable.
 
 ## Acceptance criteria
 
-- [ ] Preview opens the composed print-ready letter in a new tab; browser print
+- [x] Preview opens the composed print-ready letter in a new tab; browser print
       shows correct margins via `@page`.
-- [ ] Unresolved manual placeholders render `[NOG IN TE VULLEN: …]`; preview works
+- [x] Unresolved manual placeholders render `[NOG IN TE VULLEN: …]`; preview works
       despite lint errors (only send blocks).
-- [ ] A sent brief serves its archived HTML unchanged after an org-template republish.
+- [x] A sent brief serves its archived HTML unchanged after an org-template republish.
-- [ ] Golden + parity tests green without any browser installed.
+- [x] Golden + parity tests green without any browser installed.
-- [ ] `swagger.json` unchanged by the two endpoints (drift check green).
+- [x] `swagger.json` unchanged by the two endpoints (drift check green).
 
 ## Verification
 

src/app/brief/application/brief.store.spec.ts

@@ -1,9 +1,10 @@
 import { TestBed } from '@angular/core/testing';
-import { describe, it, expect } from 'vitest';
+import { describe, it, expect, vi, afterEach } from 'vitest';
 import { Result } from '@shared/kernel/fp';
 import { Brief, BriefDecisions } from '@brief/domain/brief';
 import { OrgTemplate } from '@brief/domain/org-template';
 import { BriefAdapter, BriefView } from '@brief/infrastructure/brief.adapter';
+import { LetterPreviewAdapter } from '@brief/infrastructure/letter-preview.adapter';
 import { BriefStore } from './brief.store';
 
 const decisions: BriefDecisions = {
@@ -103,3 +104,43 @@ describe('BriefStore action state (Idle | Busy | Failed)', () => {
     expect(store.lastError()).toBeNull();
   });
 });
+
+describe('BriefStore.previewLetter', () => {
+  // vi.spyOn reuses an existing spy (and its call history) if one is already on
+  // the property — window.open/URL.createObjectURL must be restored between tests.
+  afterEach(() => vi.restoreAllMocks());
+
+  it('opens the composed letter in a new tab on success', async () => {
+    const store = setup({
+      load: (): Promise<Result<string, BriefView>> => Promise.resolve({ ok: true, value: view }),
+    });
+    await store.load();
+    const blob = new Blob(['<html></html>'], { type: 'text/html' });
+    vi.spyOn(URL, 'createObjectURL').mockReturnValue('blob:mock');
+    const open = vi.spyOn(window, 'open').mockImplementation(() => null);
+    vi.spyOn(TestBed.inject(LetterPreviewAdapter), 'preview').mockResolvedValue({
+      ok: true,
+      value: blob,
+    });
+
+    await store.previewLetter();
+    expect(open).toHaveBeenCalledWith('blob:mock', '_blank');
+    expect(store.lastError()).toBeNull();
+  });
+
+  it('surfaces the error without opening a tab on failure', async () => {
+    const store = setup({
+      load: (): Promise<Result<string, BriefView>> => Promise.resolve({ ok: true, value: view }),
+    });
+    await store.load();
+    const open = vi.spyOn(window, 'open').mockImplementation(() => null);
+    vi.spyOn(TestBed.inject(LetterPreviewAdapter), 'preview').mockResolvedValue({
+      ok: false,
+      error: 'De voorvertoning kon niet worden geopend.',
+    });
+
+    await store.previewLetter();
+    expect(open).not.toHaveBeenCalled();
+    expect(store.lastError()).toBe('De voorvertoning kon niet worden geopend.');
+  });
+});

src/app/brief/application/brief.store.ts

@@ -12,6 +12,7 @@ import {
 import { BriefMsg, BriefState, initial, reduce } from '@brief/domain/brief.machine';
 import { OrgTemplate } from '@brief/domain/org-template';
 import { BriefAdapter, BriefView } from '@brief/infrastructure/brief.adapter';
+import { LetterPreviewAdapter } from '@brief/infrastructure/letter-preview.adapter';
 
 /** Transient action state (submit/approve/reject/send/resetDemo) — one tagged union
     instead of a busy boolean + a nullable error sitting side by side. */
@@ -35,6 +36,7 @@ type LoadedBriefState = Extract<BriefState, { tag: 'loaded' }>;
 @Injectable({ providedIn: 'root' })
 export class BriefStore {
   private adapter = inject(BriefAdapter);
+  private previewAdapter = inject(LetterPreviewAdapter);
   private store = createStore<BriefState, BriefMsg>(initial, reduce);
 
   readonly model = this.store.model;
@@ -147,6 +149,20 @@ export class BriefStore {
   reject = (comments: string) => this.transition(() => this.adapter.reject(comments));
   send = () => this.transition(() => this.adapter.send());
 
+  /** Explicit action, never a live re-render (PRD §8): opens the server-composed
+      letter in a new tab. ponytail: the blob URL is never revoked — it's cheap and
+      the tab outlives this call; not worth a teardown hook for a POC. */
+  async previewLetter() {
+    this.actionState.set({ tag: 'Busy' });
+    const r = await this.previewAdapter.preview();
+    if (!r.ok) {
+      this.actionState.set({ tag: 'Failed', error: r.error });
+      return;
+    }
+    this.actionState.set({ tag: 'Idle' });
+    window.open(URL.createObjectURL(r.value), '_blank');
+  }
+
   // A transition: flush any pending save, call the server (authoritative), then mirror
   // the returned status through the pure reducer's guarded transition.
   private async transition(action: () => Promise<Result<string, BriefView>>) {

src/app/brief/infrastructure/letter-preview.adapter.ts

@@ -0,0 +1,37 @@
+import { Injectable } from '@angular/core';
+import { Result, ok, err } from '@shared/kernel/fp';
+import { currentRole } from '@shared/infrastructure/role';
+import { problemDetail } from '@shared/infrastructure/api-error';
+import { environment } from '../../../environments/environment';
+
+const PREVIEW_FAILED = $localize`:@@brief.preview.failed:De voorvertoning kon niet worden geopend.`;
+
+/**
+ * `/brief/preview` returns `text/html`, not JSON, and is `.ExcludeFromDescription()`'d
+ * to keep the NSwag-generated client JSON-only (same seam as uploads) — so this is a
+ * hand-written fetch, not the `ApiClient`. That also means it bypasses `HttpClient`'s
+ * `roleInterceptor`, so `X-Role` is set here explicitly.
+ */
+@Injectable({ providedIn: 'root' })
+export class LetterPreviewAdapter {
+  async preview(): Promise<Result<string, Blob>> {
+    let res: Response;
+    try {
+      res = await fetch(`${environment.apiBaseUrl}/api/v1/brief/preview`, {
+        headers: { 'X-Role': currentRole() },
+      });
+    } catch {
+      return err(PREVIEW_FAILED);
+    }
+    if (!res.ok) return err(await errorMessage(res));
+    return ok(await res.blob());
+  }
+}
+
+async function errorMessage(res: Response): Promise<string> {
+  try {
+    return problemDetail(await res.json(), PREVIEW_FAILED);
+  } catch {
+    return PREVIEW_FAILED;
+  }
+}

src/app/brief/ui/brief.page.ts

@@ -63,6 +63,7 @@ import { LetterComposerComponent } from '@brief/ui/letter-composer/letter-compos
                 (approve)="store.approve()"
                 (reject)="store.reject($event)"
                 (send)="store.send()"
+                (preview)="store.previewLetter()"
               />
             }
           }

src/app/brief/ui/letter-composer/letter-composer.component.ts

@@ -41,6 +41,11 @@ import { RejectionCommentsComponent } from '@brief/ui/rejection-comments/rejecti
         flex-wrap: wrap;
         margin-block-end: var(--rhc-space-max-lg);
       }
+      .head-end {
+        display: flex;
+        align-items: center;
+        gap: var(--rhc-space-max-md);
+      }
       .panel {
         margin-block: var(--rhc-space-max-xl);
       }
@@ -56,8 +61,13 @@ import { RejectionCommentsComponent } from '@brief/ui/rejection-comments/rejecti
   template: `
     <div class="head">
       <app-heading [level]="2">{{ title() }}</app-heading>
+      <div class="head-end">
+        <app-button variant="subtle" [disabled]="busy()" (click)="preview.emit()">{{
+          previewLabel()
+        }}</app-button>
         <app-status-badge [label]="statusLabel()" [color]="statusColor()" />
       </div>
+    </div>
 
     @if (status() === 'rejected') {
       <app-rejection-comments mode="show" [comments]="rejectComments()" />
@@ -143,9 +153,11 @@ export class LetterComposerComponent {
   approve = output<void>();
   reject = output<string>();
   send = output<void>();
+  preview = output<void>();
   locate = output<Diagnostic>();
 
   title = input($localize`:@@brief.title:Brief aan de zorgverlener`);
+  previewLabel = input($localize`:@@brief.preview.open:Voorbeeld`);
   submitLabel = input($localize`:@@brief.submit:Indienen ter beoordeling`);
   resubmitLabel = input($localize`:@@brief.resubmit:Opnieuw indienen`);
   submitHint = input(