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/BeoordelingTests.cs

@@ -7,8 +7,8 @@ using Microsoft.AspNetCore.Mvc.Testing;
 
 namespace BigRegister.Tests;
 
-/// WP-65 (read side): one aanvraag's case-treatment detail, gated by the same medewerker
-/// capability (`CanBeoordelen`, WP-62) as the werkvoorraad list (WP-64).
+/// One aanvraag's case-treatment detail (read side), gated by the same medewerker
+/// capability (`CanBeoordelen`) as the werkvoorraad list.
 public class BeoordelingTests(TestWebApplicationFactory factory) : IClassFixture<TestWebApplicationFactory>
 {
   private readonly HttpClient _client = factory.CreateClient();
@@ -152,7 +152,7 @@ public class BeoordelingTests(TestWebApplicationFactory factory) : IClassFixture
       Assert.Equal("Goedgekeurd", view.Aanvraag.Status.Tag);
       Assert.False(view.Decisions.CanBesluiten); // terminal — no further decision allowed
 
-      // RB-07/BIO-007: the gate records that a behandelaar was allowed to act; this records
+      // BIO-007: the gate records that a behandelaar was allowed to act; this records
       // what they decided, which is the question /beheer/audit exists to answer.
       Assert.Contains(AuthzAuditStore.List(), e =>
           e.Action == "aanvraag:besluit" && e.Decision == "allow" &&
@@ -222,7 +222,7 @@ public class BeoordelingTests(TestWebApplicationFactory factory) : IClassFixture
     }
   }
 
-  // WP-68 (F2): the transition-legality check now runs inside RecordBesluit's write lock, so
+  // The transition-legality check now runs inside RecordBesluit's write lock, so
   // two besluiten racing on the same still-open aanvraag can't both pass the check before
   // either writes — exactly one commits, the other sees the now-terminal status.
   [Fact]