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/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>