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/mlflow/compose.yml

@@ -0,0 +1,27 @@
+# mlflow — experiment tracking + model registry (cloud only)
+# Networks: app (UI on 5000, proxied by nginx) + data (postgres backend on sql stack)
+
+services:
+  mlflow:
+    image: ghcr.io/mlflow/mlflow:v2.18.0
+    restart: unless-stopped
+    networks: [app, data]
+    command: >
+      mlflow server
+      --host 0.0.0.0
+      --port 5000
+      --backend-store-uri postgresql://${MLFLOW_DB_USER}:${MLFLOW_DB_PASSWORD}@sql:5432/${MLFLOW_DB_NAME}
+      --default-artifact-root /mlflow/artifacts
+      --serve-artifacts
+    volumes:
+      - mlflow-artifacts:/mlflow/artifacts
+    environment:
+      TZ: ${TZ:-Europe/Amsterdam}
+    # TODO: switch artifact store to S3/MinIO; Keycloak OIDC via nginx auth_request
+
+networks:
+  app:
+  data:
+
+volumes:
+  mlflow-artifacts: