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/docker-compose.yml

@@ -0,0 +1,91 @@
+# Open Notificaties (NRC) stack (S-01-c). Lean adaptation of the upstream dev compose:
+# db (PostGIS) + redis (also the Celery broker) + migrate-init + the API + a celery worker.
+#
+# Shares the `cg` network with the OpenZaak stack so the two can reach each other.
+# Run BOTH together (NRC needs OpenZaak's Autorisaties API for auth wiring):
+#
+#   docker compose -f infra/openzaak/docker-compose.yml -f infra/opennotificaties/docker-compose.yml up -d
+#   curl -s -o /dev/null -w '%{http_code}\n' http://localhost:8001/admin/   # -> 302
+#
+# NRC is published on host :8001 (OpenZaak holds :8000).
+services:
+  nrc-db:
+    image: docker.io/postgis/postgis:17-3.5
+    environment:
+      POSTGRES_USER: opennotificaties
+      POSTGRES_PASSWORD: opennotificaties
+      POSTGRES_DB: opennotificaties
+    command: postgres -c max_connections=300
+    volumes:
+      - nrc-db:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U opennotificaties -d opennotificaties"]
+      interval: 5s
+      timeout: 3s
+      retries: 10
+    networks: [cg]
+
+  nrc-redis:
+    image: docker.io/library/redis:7
+    networks: [cg]
+
+  nrc-init:
+    image: docker.io/openzaak/open-notificaties:${OPENNOTIFICATIES_TAG:-latest}
+    environment: &nrc-env
+      DJANGO_SETTINGS_MODULE: nrc.conf.docker
+      SECRET_KEY: ${NRC_SECRET_KEY:-dev-only-not-for-production}
+      DB_HOST: nrc-db
+      DB_NAME: opennotificaties
+      DB_USER: opennotificaties
+      DB_PASSWORD: opennotificaties
+      IS_HTTPS: "no"
+      ALLOWED_HOSTS: "*"
+      CACHE_DEFAULT: nrc-redis:6379/0
+      CACHE_AXES: nrc-redis:6379/0
+      CELERY_BROKER_URL: redis://nrc-redis:6379/1
+      CELERY_RESULT_BACKEND: redis://nrc-redis:6379/1
+      DISABLE_2FA: "true"
+      OPENNOTIFICATIES_SUPERUSER_USERNAME: admin
+      DJANGO_SUPERUSER_PASSWORD: admin
+      OPENNOTIFICATIES_SUPERUSER_EMAIL: admin@localhost
+      RUN_SETUP_CONFIG: "true"
+    command: /setup_configuration.sh
+    volumes:
+      - ./setup_configuration:/app/setup_configuration:ro,z
+    depends_on:
+      nrc-db:
+        condition: service_healthy
+      nrc-redis:
+        condition: service_started
+    networks: [cg]
+
+  nrc-web:
+    image: docker.io/openzaak/open-notificaties:${OPENNOTIFICATIES_TAG:-latest}
+    environment: *nrc-env
+    healthcheck:
+      test: ["CMD", "python", "-c", "import requests,sys; sys.exit(0 if requests.head('http://localhost:8000/admin/').status_code in (200,302) else 1)"]
+      interval: 10s
+      timeout: 5s
+      retries: 10
+      start_period: 30s
+    ports:
+      - "8001:8000"
+    depends_on:
+      nrc-init:
+        condition: service_completed_successfully
+    networks: [cg]
+
+  nrc-celery:
+    image: docker.io/openzaak/open-notificaties:${OPENNOTIFICATIES_TAG:-latest}
+    environment: *nrc-env
+    command: /celery_worker.sh
+    depends_on:
+      nrc-init:
+        condition: service_completed_successfully
+    networks: [cg]
+
+volumes:
+  nrc-db:
+
+networks:
+  cg: