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/device_service.py

@@ -3,7 +3,9 @@ Device Service - 设备管理服务
 Provides CRUD operations and statistics for badge devices.
 """
 
-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
@@ -158,7 +160,7 @@ async def update_device(
     for field, value in update_fields.items():
         setattr(device, field, value)
 
-    device.updated_at = datetime.now(timezone.utc)
+    device.updated_at = datetime.now(BEIJING_TZ)
     await db.flush()
     await db.refresh(device)
     return device
@@ -245,7 +247,7 @@ async def batch_update_devices(
     devices = await get_devices_by_ids(db, device_ids)
     found_map = {d.id: d for d in devices}
     update_fields = update_data.model_dump(exclude_unset=True)
-    now = datetime.now(timezone.utc)
+    now = datetime.now(BEIJING_TZ)
 
     results = []
     for device_id in device_ids: