feat(server): main.go assemblage complet + Docker + Nginx

- Assemble tous les packages (config, db, handlers, transport, websocket, prometheus)
- Boucle de rétention et détection offline toutes les minutes
- Routage REST /api/agents/, /api/config, /metrics, /ws
- Dockerfile multi-stage CGO_ENABLED=0 (alpine:3.19)
- docker-compose.yml avec service server + dashboard Nginx
- nginx.conf avec proxy WebSocket et fallback SPA

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/docker-compose.yml

@@ -0,0 +1,29 @@
+version: '3.8'
+services:
+  server:
+    build: .
+    restart: unless-stopped
+    environment:
+      UDP_ADDR: "0.0.0.0:9999"
+      DB_PATH: "/data/nanometrics.db"
+      HTTP_ADDR: "0.0.0.0:8080"
+      MQTT_BROKER: "tcp://10.0.0.3:1883"
+      MQTT_TOPIC_BASE: "nanometrics/agents"
+    volumes:
+      - nanometrics_data:/data
+    ports:
+      - "9999:9999/udp"
+
+  dashboard:
+    image: nginx:alpine
+    restart: unless-stopped
+    volumes:
+      - ./nginx/nginx.conf:/etc/nginx/conf.d/default.conf:ro
+      - ../dashboard:/usr/share/nginx/html:ro
+    ports:
+      - "80:80"
+    depends_on:
+      - server
+
+volumes:
+  nanometrics_data: