feat(openzaak): real secrets + TLS for the production OpenZaak harness (WP-55)

docker-compose.openzaak.prod.yml layers real SECRET_KEY/DB password/site
domain/allowed-hosts (all required, fail-fast via ${VAR:?...}) on top of the
WP-54 dev harness, switches Postgres off trust auth, and sets IS_HTTPS for a
front-facing reverse-proxy TLS setup. The ZGW client secret lives inside a
file setup_configuration reads rather than a compose env var, so it's
templated (data.prod.yaml.template, no secret) and rendered host-side via
render-prod-secrets.sh into a gitignored data.prod.yaml, mounted over the
container's dev data.yaml. ZgwOptions.cs already binds from IConfiguration,
so the BFF side needed no code change.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -54,3 +54,6 @@ storybook-static
 
 # WP-54: bootstrap-catalogus.sh's own record of what it seeded into a local OpenZaak run
 backend/openzaak/seeded.env
+
+# WP-55: render-prod-secrets.sh's output — the real client secret, never committed
+backend/openzaak/setup_configuration/data.prod.yaml