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

requirements.txt

@@ -9,3 +9,5 @@ httpx>=0.25.0
 fastembed>=0.3.0
 sqlite-vec>=0.1.0
 numpy
+fastapi>=0.110.0
+uvicorn[standard]>=0.27.0