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/docker-compose.openzaak.yml

@@ -1,12 +1,12 @@
-# WP-54 — a real OpenZaak to develop/test the ZGW seam against, kept OUT of the root
+# A real OpenZaak to develop/test the ZGW seam against, kept OUT of the root
 # docker-compose.yml on purpose (see backend/openzaak/README.md): OpenZaak is a full Django
 # stack (postgres + redis), heavy compared to this repo's own FE+BFF, and nobody who isn't
 # touching the ZGW slice should have to pull/boot it.
 #
 # ponytail: trimmed vs. open-zaak's own published compose — no celery/celery-beat/celery-flower
 # (async notification delivery, never asserted by the integration test) and no nginx (the test
-# hits web's port directly). Add them back only if a later WP needs an actual notification
-# round-trip against this harness (NRC delivery is already covered by fixture tests, WP-52).
+# hits web's port directly). Add them back only if a later change needs an actual notification
+# round-trip against this harness (NRC delivery is already covered by fixture tests).
 services:
   db:
     image: postgis/postgis:17-3.5