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/services/beacon_service.py

@@ -3,7 +3,9 @@ Beacon Service - 蓝牙信标管理服务
 Provides CRUD operations for Bluetooth beacon configuration.
 """
 
-from datetime import datetime, timezone
+from datetime import datetime
+
+from app.config import BEIJING_TZ
 
 from sqlalchemy import func, select, or_
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -78,7 +80,7 @@ async def update_beacon(
     update_data = data.model_dump(exclude_unset=True)
     for key, value in update_data.items():
         setattr(beacon, key, value)
-    beacon.updated_at = datetime.now(timezone.utc)
+    beacon.updated_at = datetime.now(BEIJING_TZ)
 
     await db.flush()
     await db.refresh(beacon)