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/Domain/IntakeRuleTests.cs

@@ -4,7 +4,7 @@ namespace BigRegister.Tests.Domain;
 
 public class IntakeRuleTests
 {
-  // The arguments ARE the Given (WP-69/bdd.mdx) — these degenerate to When/Then.
+  // The arguments ARE the Given (bdd.mdx) — these degenerate to When/Then.
 
   [Fact]
   public void Below_threshold_with_no_answer_is_incomplete() =>