Unify all timestamps to Beijing time (UTC+8)

- Add BEIJING_TZ constant in config.py
- Replace all timezone.utc references across 11 files
- Device-reported times, DB defaults, protocol sync all use Beijing time

via [HAPI](https://hapi.run)

Co-Authored-By: HAPI <noreply@hapi.run>

app/websocket_manager.py

@@ -6,7 +6,9 @@ Manages client connections, topic subscriptions, and broadcasting.
 import asyncio
 import json
 import logging
-from datetime import datetime, timezone
+from datetime import datetime
+
+from app.config import BEIJING_TZ
 
 from fastapi import WebSocket
 
@@ -57,7 +59,7 @@ class WebSocketManager:
             return
 
         message = json.dumps(
-            {"topic": topic, "data": data, "timestamp": datetime.now(timezone.utc).isoformat()},
+            {"topic": topic, "data": data, "timestamp": datetime.now(BEIJING_TZ).isoformat()},
             default=str,
             ensure_ascii=False,
         )