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/openzaak/bootstrap-notificaties.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
-# WP-58 — points OpenZaak's own NotificationsConfig straight at this repo's BFF webhook
-# (POST /api/v1/zgw/notificaties, WP-52) instead of standing up a real Notificaties API (NRC)
+# Points OpenZaak's own NotificationsConfig straight at this repo's BFF webhook
+# (POST /api/v1/zgw/notificaties) instead of standing up a real Notificaties API (NRC)
 # + abonnement — see docker-compose.openzaak.notificaties.yml's ponytail note for why. Requires
 # that overlay running (adds the celery worker + flips NOTIFICATIONS_DISABLED) AND the repo
 # root's own `docker compose up` running (the overlay joins its `api` container's network —
@@ -41,7 +41,7 @@ from zgw_consumers.models import Service
 service, _ = Service.objects.update_or_create(
     slug="bff-webhook",
     defaults=dict(
-        label="BIG-register BFF webhook (WP-58)",
+        label="BIG-register BFF webhook",
         api_type=APITypes.orc,
         api_root="$BFF_API_ROOT",
         auth_type=AuthTypes.api_key,