feat: Phase 3 & 4 — monitoring, discovery, polish, deployment

Phase 3 — Discovery & Monitoring:
- Network scanner: nmap wrapper + mock fallback, fingerprint service (35 signatures)
- Status checker: ping/http/https/tcp/ssh/prometheus/health per-node checks
- APScheduler: status checks every 60s, WebSocket broadcast
- WebSocket /ws/status: live node status updates to frontend
- Sidebar panels: Pending Devices, Hidden Devices, Scan History
- Auth token persisted to localStorage (survive page refresh)
- 24 new backend tests (scan flow + status_checker)

Phase 4 — Polish & Deployment:
- Auto-layout: Dagre hierarchical TB via Toolbar button
- Export PNG: html-to-image download via Toolbar button
- Scan config modal: CIDR ranges + check interval, GET/POST /api/v1/scan/config
- Dockerfile.backend (Python 3.13 slim + nmap), Dockerfile.frontend (nginx)
- docker-compose.yml with data volume and NET_RAW cap for ping
- scripts/lxc-install.sh: Proxmox VE systemd bootstrap
- README.md: quick-start, config reference, stack overview

frontend/package-lock.json

@@ -9,6 +9,7 @@
       "version": "0.0.0",
       "dependencies": {
         "@base-ui/react": "^1.2.0",
+        "@dagrejs/dagre": "^2.0.4",
         "@fontsource-variable/geist": "^5.2.8",
         "@fontsource-variable/inter": "^5.2.8",
         "@fontsource/jetbrains-mono": "^5.2.8",
@@ -799,6 +800,21 @@
         "node": ">=20.19.0"
       }
     },
+    "node_modules/@dagrejs/dagre": {
+      "version": "2.0.4",
+      "resolved": "https://registry.npmjs.org/@dagrejs/dagre/-/dagre-2.0.4.tgz",
+      "integrity": "sha512-J6vCWTNpicHF4zFlZG1cS5DkGzMr9941gddYkakjrg3ZNev4bbqEgLHFTWiFrcJm7UCRu7olO3K6IRDd9gSGhA==",
+      "license": "MIT",
+      "dependencies": {
+        "@dagrejs/graphlib": "3.0.4"
+      }
+    },
+    "node_modules/@dagrejs/graphlib": {
+      "version": "3.0.4",
+      "resolved": "https://registry.npmjs.org/@dagrejs/graphlib/-/graphlib-3.0.4.tgz",
+      "integrity": "sha512-HxZ7fCvAwTLCWCO0WjDkzAFQze8LdC6iOpKbetDKHIuDfIgMlIzYzqZ4nxwLlclQX+3ZVeZ1K2OuaOE2WWcyOg==",
+      "license": "MIT"
+    },
     "node_modules/@dotenvx/dotenvx": {
       "version": "1.54.1",
       "resolved": "https://registry.npmjs.org/@dotenvx/dotenvx/-/dotenvx-1.54.1.tgz",