feat(infra): Open Notificaties up + shared network with OpenZaak (closes #2)

Add infra/opennotificaties/docker-compose.yml (PostGIS db, redis broker,
migrate-init, API, celery worker) on a shared `cg` network with the
OpenZaak stack (renamed oz->cg), published on host :8001. Add combined
`make stack-up/stack-smoke/stack-down` targets and a runbook.

Completes S-01's acceptance: OpenZaak + Open Notificaties + Postgres come
up in compose and a health check confirms all reachable (OZ 403/302/200,
NRC 302; cross-network DNS verified). Catalogus seed landed in S-01-b.

OZ->NRC notification *delivery* wiring (NRC setup_configuration: Services +
Authorization + JWT + Kanalen + Abonnementen; OZ NotificationConfig; re-
enabling NOTIFICATIONS_DISABLED) is deferred to S-06 (Event Subscriber),
the slice that consumes events. NRC data.yaml is migrate-only until then.

Verified: `make stack-smoke` green (both platforms reachable).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/runbooks/openzaak.md

@@ -66,8 +66,9 @@ The Makefile auto-points `DOCKER_HOST` at the Podman socket when it exists, so t
 - **Not in `make ci`.** The OpenZaak smoke is a separate, heavier check (large image
   pull + migrations); it is intentionally kept out of `make ci` so the core gate
   stays fast. Run `make openzaak-smoke` when you touch the OpenZaak stack.
-- **Notifications disabled.** `NOTIFICATIONS_DISABLED=true` until Open Notificaties
-  (NRC) lands in S-01-c — otherwise ZTC writes 500 trying to notify. Re-enable then.
+- **Notifications disabled.** `NOTIFICATIONS_DISABLED=true` — otherwise ZTC writes 500
+  trying to notify. Open Notificaties is now up (see [opennotificaties.md](opennotificaties.md)),
+  but OZ→NRC delivery wiring + re-enabling lands with **S-06**.
 - **Zaaktype is a concept**, not published (publishing needs roltypen/statustypen/
   resultaattypen — beyond the lean seed). List with `?status=alles`.
 - **Image tag.** Currently `openzaak/open-zaak:latest` via `${OPENZAAK_TAG}`; pin to