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/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");