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/mosquitto/.env.example

@@ -0,0 +1 @@
+# mosquitto — broker uses config file, no env vars in stub

stacks/mosquitto/README.md

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+# mosquitto
+
+Eclipse Mosquitto MQTT broker. **Reserved for the FROST (SensorThings API) stack** — separate from the general-purpose `rabbitmq` broker. **Cloud-only.**
+
+- **Network**: `app` (internal only — FROST services connect via service name `mosquitto`)
+- **No external ingress** by default. If FROST needs external MQTT publishers, route them through a separate nginx stream block on a different port (not 8883 — that belongs to rabbitmq).
+- **Config**: `config/mosquitto.conf` — listener config, ACLs, persistence
+- **TODO**:
+  - ACL aligned with FROST topic structure
+  - Persistence retention policy
+  - Optional shovel from `rabbitmq` if cross-broker forwarding is needed

stacks/mosquitto/compose.yml

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+# mosquitto — MQTT broker reserved for the FROST (SensorThings) stack
+# Cloud-only. Internal to its own stack; no external ingress by default.
+# Networks: app
+
+services:
+  mosquitto:
+    image: eclipse-mosquitto:2.0
+    restart: unless-stopped
+    networks: [app]
+    volumes:
+      - ./config/mosquitto.conf:/mosquitto/config/mosquitto.conf:ro
+      - mosquitto-data:/mosquitto/data
+      - mosquitto-log:/mosquitto/log
+    # No 'ports:' — FROST is the only intended consumer. If external MQTT
+    # access for FROST is needed later, add a separate nginx stream block.
+
+networks:
+  app:
+
+volumes:
+  mosquitto-data:
+  mosquitto-log: