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/nginx/nginx.conf

@@ -0,0 +1,22 @@
+server {
+    listen 80;
+    root /usr/share/nginx/html;
+    index index.html;
+
+    location /api/ {
+        proxy_pass http://server:8080;
+        proxy_set_header Host $host;
+    }
+    location /ws {
+        proxy_pass http://server:8080/ws;
+        proxy_http_version 1.1;
+        proxy_set_header Upgrade $http_upgrade;
+        proxy_set_header Connection "upgrade";
+    }
+    location /metrics {
+        proxy_pass http://server:8080/metrics;
+    }
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+}