scaffold: hub-and-spoke layout, 4-network topology, 13 stack stubs

Initial structure for R&D infrastructure:

- stacks/ — 13 reusable, runnable stack stubs (kebab-case)
  cloud-and-edge: node-red, influxdb, grafana, keycloak, portainer,
                  nginx-proxy, mqtt, postfix
  cloud-only:     wireguard-server, gitea, jenkins, sql (postgres stub)
  edge-only:      wireguard-client

- cloud/ — single central hub composition with 4 networks
           (edge, app, data internal, mgmt) and include: stubs
- sites/ — per-plant edge folders (template README only for now)
- docs/architecture.md — hub-and-spoke + ingress + segmentation rationale

Network model: only nginx-proxy (80/443/8883) and wireguard-server
(51820/udp) publish ports on the cloud host. Edge nginx publishes
80/443 on plant-LAN interface only. MQTT cloud-side via nginx stream
proxy; MQTT edge-side internal-only; Postfix outbound-only.

OT layer (OPCUA, PLCs) is out of scope for this repo.

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

stacks/postfix/compose.yml

@@ -0,0 +1,16 @@
+# postfix — outbound mail relay only (no inbound, no published port)
+# Networks: app
+
+services:
+  postfix:
+    image: boky/postfix:4.2.0
+    restart: unless-stopped
+    networks: [app]
+    environment:
+      RELAYHOST: ${POSTFIX_RELAYHOST}
+      ALLOWED_SENDER_DOMAINS: ${POSTFIX_FROM_DOMAIN}
+      TZ: ${TZ:-Europe/Amsterdam}
+    # No 'ports:' — outbound only. Internal services hit smtp://postfix:25 on the app network.
+
+networks:
+  app: