feat: implement graceful shutdown with SIGINT/SIGTERM support

- Add WebSocket shutdown() with client notification and graceful close
- Enhance download service stop() with pending state persistence
- Expand FastAPI lifespan shutdown with proper cleanup sequence
- Add SQLite WAL checkpoint before database close
- Update stop_server.sh to use SIGTERM with timeout fallback
- Configure uvicorn timeout_graceful_shutdown=30s
- Update ARCHITECTURE.md with shutdown documentation

data/config.json

@@ -17,7 +17,7 @@
     "keep_days": 30
   },
   "other": {
-    "master_password_hash": "$pbkdf2-sha256$29000$8P7fG2MspVRqLaVUyrn3Pg$e0HxlEoo7eAfETUFCi7G4/0egtE.Foqsf9eR69Dg6a0"
+    "master_password_hash": "$pbkdf2-sha256$29000$cC6FsJayNmYsZezdW6tVyg$5LMyYrqVoM0qwxugSedT6UFMnLHePg2atdECBxAVJEk"
   },
   "version": "1.0.0"
 }