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/OpenZaakZaakSourceTests.cs

@@ -182,7 +182,7 @@ public class OpenZaakZaakSourceTests
     Assert.Throws<InvalidOperationException>(() => source.CreateZaak(aanvraag, DateTimeOffset.UtcNow, caller));
   }
 
-  // --- WP-66: besluit write (a status transition on an existing zaak) --------------------
+  // --- Besluit write (a status transition on an existing zaak) --------------------
 
   [Fact]
   public void RecordBesluit_posts_the_last_statustype_with_besluit_and_toelichting()
@@ -308,7 +308,7 @@ public class OpenZaakZaakSourceTests
     Assert.Throws<InvalidOperationException>(() => source.RecordBesluit(aanvraag, Besluit.Goedkeuren, null, DateTimeOffset.UtcNow, caller));
   }
 
-  // --- WP-60: bounded retry in ZgwHttpClient, exercised through the create-zaak write path ---
+  // --- Bounded retry in ZgwHttpClient, exercised through the create-zaak write path ---
 
   private static (ZgwOptions options, Aanvraag.Submitted aanvraag, CallerIdentity caller) CreateZaakFixture()
   {