feat: SQL=postgres, nginx+certbot, MQTT split, ML stacks, gitea HTTPS-only, gemaal1 site

Round-2 changes locking in scaffold-phase decisions and adding ML/notebook stacks.

Locked decisions
- sql: postgres 16-alpine (was TBD); init.d/ mount for per-app DB provisioning
- nginx-proxy: stock nginx + certbot sidecar (was nginx:alpine TODO).
  Chose stock over nginxproxy/nginx-proxy because stream{} is required for
  MQTT-TLS reverse-proxy on tcp/8883 to rabbitmq:1883.
- gitea: HTTPS-only (DISABLE_SSH=true). No SSH port published.

MQTT split
- Remove stacks/mqtt placeholder.
- Add stacks/rabbitmq — general-purpose broker (AMQP + MQTT plugin),
  used at both cloud and edge. External MQTT clients reach cloud broker
  via nginx stream-proxy on 8883.
- Add stacks/mosquitto — reserved for the FROST (SensorThings) stack
  only. Cloud-only. Internal to its own stack; no external ingress.

ML / notebooks (cloud-only)
- stacks/mlflow — experiment tracking + model registry. Postgres backend
  on sql stack; local volume for artifacts (S3/MinIO is a TODO).
- stacks/jupyterhub — multi-user notebook server. DockerSpawner via
  mounted docker.sock; users spawn into cloud-app network so they can
  reach mlflow, influxdb (via grafana), rabbitmq.

Sites
- sites/gemaal1 — first edge deployment scaffold. Site-local override
  template for binding nginx to PLANT_LAN_IP.

Docs
- README + docs/architecture.md updated: stacks table now lists 15 stacks,
  ingress + attachment tables reflect mlflow/jupyterhub, TLS strategy
  section locked, MQTT-split section added, Gitea HTTPS-only noted.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

stacks/rabbitmq/README.md

@@ -0,0 +1,17 @@
+# rabbitmq
+
+General-purpose message broker. AMQP for app-to-app messaging; MQTT plugin for external MQTT-TLS clients (fronted by nginx-proxy stream block).
+
+Used at both cloud and edge. The `mosquitto` stack is reserved for the FROST SensorThings stack only — do not confuse the two.
+
+- **Network**: `app` — no published port
+- **External MQTT** clients reach `nginx-proxy:8883` (cloud) which stream-proxies to `rabbitmq:1883` internally. Edge brokers are internal-only.
+- **Management UI**: port 15672 → reverse-proxied through nginx-proxy
+- **Plugins to enable** in `config/enabled_plugins`:
+  ```
+  [rabbitmq_management, rabbitmq_mqtt, rabbitmq_web_mqtt].
+  ```
+- **TODO**:
+  - ACL policies
+  - VHost layout (cloud-bus, plant-buses, ml-bus)
+  - Shovel/federation to `mosquitto` if FROST needs MQTT bridge to general broker