refactor: strip WP-/RB- ticket refs from backend (RD-19)

The backend half of the sweep RD-18 did for the front end. git blame
holds the provenance and stays correct when the code moves; the
comment names a closed ticket and tells the reader nothing the
sentence around it does not.

public/letter.css and LetterHtml.golden.html change together, because
the renderer inlines the CSS and the golden file snapshots the
result.

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

backend/tests/BigRegister.Tests/FeatureFlagTests.cs

@@ -6,7 +6,7 @@ using Microsoft.AspNetCore.Mvc.Testing;
 
 namespace BigRegister.Tests;
 
-/// WP-47: runtime feature flags — catalog in code, admin-toggled, server-enforced.
+/// Runtime feature flags — catalog in code, admin-toggled, server-enforced.
 public class FeatureFlagTests(TestWebApplicationFactory factory) : IClassFixture<TestWebApplicationFactory>
 {
   private readonly HttpClient _client = factory.CreateClient();