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/WerkvoorraadTests.cs

@@ -5,8 +5,8 @@ using Microsoft.AspNetCore.Mvc.Testing;
 
 namespace BigRegister.Tests;
 
-/// WP-64: the behandelportal's queue of aanvragen needing treatment, gated by the
-/// medewerker capability `CanBeoordelen` (WP-62) — not the admin role.
+/// The behandelportal's queue of aanvragen needing treatment, gated by the
+/// medewerker capability `CanBeoordelen` — not the admin role.
 public class WerkvoorraadTests(TestWebApplicationFactory factory) : IClassFixture<TestWebApplicationFactory>
 {
   private readonly HttpClient _client = factory.CreateClient();
@@ -38,7 +38,7 @@ public class WerkvoorraadTests(TestWebApplicationFactory factory) : IClassFixtur
       var queue = (await res.Content.ReadFromJsonAsync<List<AanvraagSummaryDto>>())!;
       var mine = queue.Single(x => x.Id == a.Id);
       Assert.Equal("InBehandeling", mine.Status.Tag);
-      // RB-03/BIO-003: masked, like /admin/cases — both inherit ToAdminSummaryDto.
+      // BIO-003: masked, like /admin/cases — both inherit ToAdminSummaryDto.
       Assert.Equal("******782", mine.Owner);
     }
     finally