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/src/BigRegister.Api/Data/DocumentStore.cs

@@ -4,7 +4,7 @@ namespace BigRegister.Api.Data;
 
 /// <summary>
 /// Stored document: metadata + bytes. The demo persists bytes in the SQLite file
-/// (WP-22) purely so a re-opened wizard can preview/download what was uploaded — a
+/// purely so a re-opened wizard can preview/download what was uploaded — a
 /// real backend persists them to blob storage keyed by DocumentId. Bytes are never
 /// serialized into a JSON response; only the dedicated content endpoint streams them.
 /// </summary>
@@ -14,7 +14,7 @@ public sealed record StoredDocument(
 {
   public bool Linked { get; set; }
 
-  /// <summary>The OpenZaak DRC enkelvoudiginformatieobject's URL, set once Upload (WP-51)
+  /// <summary>The OpenZaak DRC enkelvoudiginformatieobject's URL, set once Upload
   /// registers one — null under the local source. Persisted so the later zaak-link step can
   /// find it without re-uploading; not part of the positional constructor, same reasoning as
   /// <see cref="Linked"/> (every existing `new StoredDocument(...)` call site keeps working).</summary>
@@ -30,7 +30,7 @@ public sealed record AuditEntry(DateTimeOffset At, string Action, string Documen
 }
 
 /// <summary>
-/// EF Core/SQLite-backed document store + audit log (WP-22 — was a static
+/// EF Core/SQLite-backed document store + audit log (was a static
 /// Dictionary). ponytail: one global lock, same as before — SQLite tolerates only
 /// one writer at a time anyway, and this process already serialized all access
 /// through a single gate, so it now doubles as a coarse single-writer guard for
@@ -85,7 +85,7 @@ public static class DocumentStore
     }
   }
 
-  /// <summary>Documents by DocumentId (WP-65's beoordeling detail reads an aanvraag's already-
+  /// <summary>Documents by DocumentId (the beoordeling detail reads an aanvraag's already-
   /// linked documents) — the DocumentId-keyed counterpart of <see cref="ByLocalIds"/>, which is
   /// keyed by the wizard's own LocalId instead.</summary>
   public static IReadOnlyList<StoredDocument> ByIds(IEnumerable<string> documentIds)
@@ -114,7 +114,7 @@ public static class DocumentStore
     }
   }
 
-  /// <summary>Persist the DRC url an OpenZaak upload (WP-51) registered for a document.</summary>
+  /// <summary>Persist the DRC url an OpenZaak upload registered for a document.</summary>
   public static void SetDrcUrl(string documentId, string drcUrl)
   {
     lock (_gate)
@@ -181,7 +181,7 @@ public static class DocumentStore
   }
 
   /// <summary>Append one metadata-only audit row. <paramref name="actor"/> must arrive
-  /// **already redacted** (RB-04/BIO-005) — the two citizen call sites pass
+  /// **already redacted** (BIO-005) — the two citizen call sites pass
   /// <see cref="Pii.MaskTail"/> of the owner BSN, `delete-admin` passes the literal
   /// `"admin"`. The unmasked BSN lives only in <see cref="StoredDocument.Owner"/>, which is
   /// the authorization key and stays untouched. Masking here instead would have to guess