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/ZgwTokenProviderTests.cs

@@ -59,7 +59,7 @@ public class ZgwTokenProviderTests
   [Fact]
   public void Mint_with_a_medewerker_caller_uses_the_medewerkerId_as_user_id()
   {
-    // WP-62: SubjectId is what ZgwTokenProvider.Mint reads — a medewerker's is its
+    // SubjectId is what ZgwTokenProvider.Mint reads — a medewerker's is its
     // medewerkerId, not a BSN, and this is the only place that's directly observable.
     var caller = new MedewerkerCaller("m.jansen", [MedewerkerRol.Behandelaar], "M. Jansen", PrincipalRole.Drafter);
     var token = new ZgwTokenProvider(Options).Mint(caller);