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/BriefEndpointTests.cs

@@ -28,7 +28,7 @@ public class BriefEndpointTests(TestWebApplicationFactory factory) : IClassFixtu
     return new SaveBriefRequest(sections);
   }
 
-  /// RB-23: `GET /brief` no longer seeds a brief on first call, so every test that
+  /// `GET /brief` no longer seeds a brief on first call, so every test that
   /// needs one present creates it explicitly through `POST /brief/reset`
   /// (`BriefStore.ResetAndCreate`) — the same command the "start over" affordance uses.
   private async Task<BriefDto> SeedBrief()
@@ -48,7 +48,7 @@ public class BriefEndpointTests(TestWebApplicationFactory factory) : IClassFixtu
     return req;
   }
 
-  // --- RB-23/CQ-007: GET /brief is a pure query — it must not create a row. ---
+  // --- CQ-007: GET /brief is a pure query — it must not create a row. ---
 
   [Fact]
   public async Task Get_returns_404_and_writes_no_row_when_no_brief_exists_for_the_owner()
@@ -181,8 +181,8 @@ public class BriefEndpointTests(TestWebApplicationFactory factory) : IClassFixtu
     Assert.NotNull(submitted);
     Assert.Equal("submitted", submitted.Brief.Status.Tag);
 
-    // RB-07/BIO-007: the allow side of the transition leaves a row, not just a log line.
-    // Resource is the bare "brief" — never the owner's BSN (RB-02).
+    // BIO-007: the allow side of the transition leaves a row, not just a log line.
+    // Resource is the bare "brief" — never the owner's BSN.
     Assert.Contains(AuthzAuditStore.List(),
         e => e.Action == "brief:submit" && e.Decision == "allow" && e.Resource == "brief");
   }