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/OpenZaakIntegrationTests.cs

@@ -8,7 +8,7 @@ using Microsoft.AspNetCore.Mvc.Testing;
 namespace BigRegister.Tests;
 
 /// <summary>
-/// WP-54: the one test that proves the BFF actually talks to a REAL OpenZaak — auth accepted,
+/// The one test that proves the BFF actually talks to a REAL OpenZaak — auth accepted,
 /// real response shapes, real pagination/zaaktype→aanvraag-type mapping — rather than the stub
 /// HttpMessageHandler every other Zgw test (<see cref="ZgwZaakMapperTests"/>,
 /// <see cref="OpenZaakZaakSourceTests"/>) uses. Requires the harness in <c>backend/openzaak/</c>
@@ -36,7 +36,7 @@ public class OpenZaakIntegrationTests
         .UseSetting("Zgw:ClientId", "bigregister-test")
         .UseSetting("Zgw:Secret", "bigregister-test-secret")
         .UseSetting("Zgw:UserId", "bigregister-test")
-        .UseSetting("Zgw:UserRepresentation", "WP-54 integration test")
+        .UseSetting("Zgw:UserRepresentation", "OpenZaak integration test")
         .UseSetting("Zgw:ZaaktypeUrls:herregistratie", zaaktypeUrl));
   }