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/setup_configuration/data.yaml

@@ -2,7 +2,7 @@
 # documented CLI config mechanism — see docker-compose.openzaak.yml) instead of the Django
 # admin. Creates the ONE application the bootstrap script + integration test authenticate as.
 #
-# heeft_alle_autorisaties is false (WP-57, least privilege) — but
+# heeft_alle_autorisaties is false (least privilege) — but
 # `ApplicatieConfigurationModel` (vng_api_common's setup_configuration step) has no field for
 # granular `autorisaties` at all, only this boolean. So this client starts with ZERO scopes;
 # bootstrap-catalogus.sh grants the exact ones it needs via `manage.py shell` (Django ORM,
@@ -12,7 +12,7 @@ sites_config_enable: true
 sites_config:
   items:
     - domain: localhost:8000
-      name: OpenZaak (WP-54 harness)
+      name: OpenZaak (harness)
 
 vng_api_common_credentials_config_enable: true
 vng_api_common_credentials:
@@ -26,5 +26,5 @@ vng_api_common_applicaties:
     - uuid: 5a09b3c9-6a54-4b2b-8f3c-1f9b6b6a3a01
       client_ids:
         - bigregister-test
-      label: BIG-register BFF (WP-54 test harness)
+      label: BIG-register BFF (test harness)
       heeft_alle_autorisaties: false