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/ApplicationRuleTests.cs

@@ -4,9 +4,9 @@ namespace BigRegister.Tests.Domain;
 
 public class ApplicationRuleTests
 {
-  // WP-63: the published lifecycle (ADR-0002) must name exactly these five tags, in this
+  // The published lifecycle (ADR-0002) must name exactly these five tags, in this
   // order — ToStatusDto's string literals must keep matching Enum.ToString(), and Ingediend/
-  // MeerInfoGevraagd (unreachable until WP-65 adds the behandelaar transition) stay defined.
+  // MeerInfoGevraagd (unreachable until a later change adds the behandelaar transition) stay defined.
   [Fact]
   public void AanvraagStatusTag_covers_the_published_lifecycle()
   {