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.bff.yml

@@ -13,7 +13,7 @@
 #
 # 1. Why container-to-container instead of `http://localhost:8000`: this dev environment's
 #    rootless Podman drops container→host-port traffic through `host.docker.internal`
-#    (confirmed for the WP-58 notifications overlay's celery worker — DNS resolves it, every
+#    (confirmed for the notifications overlay's celery worker — DNS resolves it, every
 #    TCP connect times out).
 #
 # 2. Why the ROOT project's `api` joins INTO this project's network (below), not the other way