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>

infra/opennotificaties/setup_configuration/data.yaml

@@ -0,0 +1,5 @@
+# Open Notificaties setup_configuration.
+# Stage 1 (this commit): intentionally minimal — the init container runs
+# migrations; no steps enabled yet. The OpenZaak<->NRC notification wiring
+# (Services, Authorization, JWT, Kanalen) is added next. See ADR-0002 / S-01-c.
+{}