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/AdminCasesTests.cs

@@ -6,7 +6,7 @@ using Microsoft.AspNetCore.Mvc.Testing;
 
 namespace BigRegister.Tests;
 
-/// WP-36: admin cross-owner case list + admin delete, gated by `cases:manage`.
+/// Admin cross-owner case list + admin delete, gated by `cases:manage`.
 public class AdminCasesTests(TestWebApplicationFactory factory) : IClassFixture<TestWebApplicationFactory>
 {
   private readonly HttpClient _client = factory.CreateClient();
@@ -35,7 +35,7 @@ public class AdminCasesTests(TestWebApplicationFactory factory) : IClassFixture<
       list.EnsureSuccessStatusCode();
       var cases = (await list.Content.ReadFromJsonAsync<List<AanvraagSummaryDto>>())!;
       var mine = cases.Single(x => x.Id == a.Id);
-      // RB-03/BIO-003: the owner is carried, but masked — it is a BSN, and this list is
+      // BIO-003: the owner is carried, but masked — it is a BSN, and this list is
       // read by someone who is not the subject.
       Assert.Equal("******782", mine.Owner);
       Assert.DoesNotContain(DocumentStore.DemoOwner, mine.Owner);