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/AuthzTests.cs

@@ -82,7 +82,7 @@ public class AuthzTests
     Assert.False(Authz.Decisions(Approver, "draft", DrafterId).CanRevealBigNummer);
   }
 
-  // --- CanBeoordelen (WP-62) --------------------------------------------------------------
+  // --- CanBeoordelen --------------------------------------------------------------
 
   [Fact]
   public void CanBeoordelen_true_for_a_medewerker_with_the_behandelaar_rol()