feat(dashboard): embedded FastAPI backend with REST API + WebSocket

- FastAPI runs in MeshAI asyncio loop (no separate process)
- REST API: /api/status, /api/health, /api/nodes, /api/edges,
  /api/regions, /api/sources, /api/config, /api/alerts
- WebSocket at /ws/live pushes health updates and alerts
- Config CRUD: GET/PUT per section with validation and save
- DashboardConfig with port/host in config.yaml

docker-compose.yml

@@ -35,6 +35,8 @@ services:
     ports:
       # Web-based config interface (ttyd)
       - "7682:7682"
+      # Dashboard API
+      - "8080:8080"
 
     volumes:
       # Persistent data (database, config)