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/Domain/Applications/AanvraagStatus.cs

@@ -1,11 +1,11 @@
 namespace BigRegister.Domain.Applications;
 
 /// <summary>
-/// The post-submission aanvraag status lifecycle (ADR-0002, WP-63): Ingediend → In
+/// The post-submission aanvraag status lifecycle (ADR-0002): Ingediend → In
 /// behandeling → (Meer info gevraagd ⇄) → Goedgekeurd/Afgewezen. Concept (pre-submission,
 /// the wizard draft) is deliberately NOT a member here — see <see cref="AanvraagStatus.Tag"/>,
 /// which is null exactly when the aanvraag hasn't been submitted yet, instead of a sixth
-/// "magic string" tag with no enum member to match it (WP-68 F3).
+/// "magic string" tag with no enum member to match it.
 /// <see cref="Ingediend"/> is reserved: no endpoint sets it yet (there is no state between
 /// "just submitted" and "in behandeling" in this POC) — kept because the FE's status union
 /// and $localize catalogue already declare it, and removing it would ripple into both.
@@ -13,13 +13,13 @@ namespace BigRegister.Domain.Applications;
 public enum AanvraagStatusTag { Ingediend, InBehandeling, MeerInfoGevraagd, Goedgekeurd, Afgewezen }
 
 /// <summary>
-/// A behandelaar's recorded decision (WP-65b) — the three actions the beoordeling screen
+/// A behandelaar's recorded decision — the three actions the beoordeling screen
 /// offers, each advancing an aanvraag's <see cref="AanvraagStatus"/>.
 /// </summary>
 public enum Besluit { Goedkeuren, Afwijzen, MeerInfoOpvragen }
 
 /// <summary>
-/// The domain projection of an aanvraag's status at a point in time (WP-68 F3) — the type
+/// The domain projection of an aanvraag's status at a point in time — the type
 /// <c>Aanvraag.StatusAt(now)</c> returns, replacing the logic that used to live directly in
 /// <c>Contracts.Mappers.ToStatusDto</c>. Constructible only via the factories below, so a
 /// caller can never build e.g. a Referentie-less Goedgekeurd. <see cref="Tag"/> is null only