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/Acceptance/BesluitLifecycleTests.cs

@@ -9,8 +9,8 @@ using BigRegister.Tests.Builders;
 namespace BigRegister.Tests.Acceptance;
 
 /// <summary>
-/// Behaviour-level tests for the besluit lifecycle (WP-65b/66/68), built through the
-/// <see cref="Given"/> type-state builder (WP-70) rather than the full wizard/upload dance
+/// Behaviour-level tests for the besluit lifecycle, built through the
+/// <see cref="Given"/> type-state builder rather than the full wizard/upload dance
 /// <see cref="BeoordelingTests"/> uses — a fixture that's already Submitted (or already
 /// Decided) is a two-line Given, not fifteen. Each test persists its own Given-built
 /// <see cref="Aanvraag"/> straight into the isolated per-class SQLite file (no HTTP round trip