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/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);
+  }
+}