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/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+// Proefbrief: the admin's unpublished draft template rendered over a fixture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.