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/ZgwDivergenceTests.cs

@@ -9,7 +9,7 @@ using Microsoft.Extensions.DependencyInjection;
 namespace BigRegister.Tests;
 
 /// <summary>
-/// WP-60's required verification: a ZGW failure mid-submit must not leave the two write sides
+/// The required verification: a ZGW failure mid-submit must not leave the two write sides
 /// silently diverged — it's flagged (<see cref="Aanvraag.ZgwError"/>, an audit row) instead.
 /// Not an <see cref="IClassFixture{TFixture}"/> off <see cref="TestWebApplicationFactory"/>: that
 /// fixture hardcodes <c>Zgw:Enabled=false</c>, so this builds its own factory the same way
@@ -111,7 +111,7 @@ public class ZgwDivergenceTests
     Assert.Null(stored.ZgwError);
   }
 
-  /// RB-05/BIO-009: `ZgwError` is persisted to SQLite and written to the application log, so
+  /// BIO-009: `ZgwError` is persisted to SQLite and written to the application log, so
   /// the message it carries may not include the response body (OpenZaak echoes the request in
   /// its errors) or the request's query string (ZGW filters travel there, and one of them is
   /// `rol__betrokkeneIdentificatie__natuurlijkPersoon__inpBsn`).