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/StamdataEndpointTests.cs

@@ -59,7 +59,7 @@ public class StamdataEndpointTests(TestWebApplicationFactory factory) : IClassFi
     Assert.Empty(table.Rows);
   }
 
-  /// RB-16/BIO-019: DateOnly.Parse used to throw FormatException on unparseable input,
+  /// BIO-019: DateOnly.Parse used to throw FormatException on unparseable input,
   /// surfacing as an unhandled 500 instead of the 400-with-problem-details every other
   /// bad-input check in this endpoint file returns.
   [Fact]