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/IntakeSubmissionTests.cs

@@ -8,8 +8,8 @@ using BigRegister.Tests.Builders;
 namespace BigRegister.Tests.Acceptance;
 
 /// <summary>
-/// Behaviour-level tests for the scholing-threshold enforcement (WP-69) over
-/// <c>POST /aanvragen/{id}/submit</c> (the wizard's real path — WP-72 deleted the legacy
+/// Behaviour-level tests for the scholing-threshold enforcement over
+/// <c>POST /aanvragen/{id}/submit</c> (the wizard's real path — a later change deleted the legacy
 /// <c>POST /intakes</c> endpoint this once also covered). Built through the <see
 /// cref="Given"/> type-state builder, mirroring <see cref="BesluitLifecycleTests"/> rather
 /// than the full wizard/upload dance — the builder's default owner IS <see