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/main.py

@@ -28,11 +28,12 @@ from app.extensions import limiter
 
 async def run_data_cleanup():
     """Delete records older than DATA_RETENTION_DAYS."""
-    from datetime import datetime, timezone, timedelta
+    from datetime import datetime, timedelta
+    from app.config import BEIJING_TZ
     from sqlalchemy import delete
     from app.models import LocationRecord, HeartbeatRecord, AlarmRecord, AttendanceRecord, BluetoothRecord
 
-    cutoff = datetime.now(timezone.utc) - timedelta(days=settings.DATA_RETENTION_DAYS)
+    cutoff = datetime.now(BEIJING_TZ) - timedelta(days=settings.DATA_RETENTION_DAYS)
     total_deleted = 0
     async with async_session() as session:
         async with session.begin():