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/UploadAccessTests.cs

@@ -7,10 +7,10 @@ using Microsoft.AspNetCore.Mvc.Testing;
 
 namespace BigRegister.Tests;
 
-/// Who may see what about an upload. RB-01/BIO-004: GET /uploads/{id}/content and
+/// Who may see what about an upload. BIO-004: GET /uploads/{id}/content and
 /// /uploads/status used to take no HttpContext at all — a diploma or identity scan was
 /// protected by GUID unguessability alone, while DELETE on the same resource was
-/// owner-scoped. RB-04/BIO-005: the document audit trail recorded the raw owner BSN as
+/// owner-scoped. BIO-005: the document audit trail recorded the raw owner BSN as
 /// its Actor, on a store whose own doc comment says it holds no PII.
 public class UploadAccessTests(TestWebApplicationFactory factory) : IClassFixture<TestWebApplicationFactory>
 {