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/render-prod-secrets.sh

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-# WP-55 — renders setup_configuration/data.prod.yaml.template into the gitignored
+# Renders setup_configuration/data.prod.yaml.template into the gitignored
 # data.prod.yaml docker-compose.openzaak.prod.yml mounts over the container's data.yaml.
 # Run this once before `docker compose ... up` in a production deploy; re-run whenever the
 # secrets rotate. Fails fast (no output file) if a required env var is missing — never