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/dependencies.py

@@ -5,7 +5,9 @@ Supports master API key (env) and database-managed API keys with permission leve
 
 import hashlib
 import secrets
-from datetime import datetime, timezone
+from datetime import datetime
+
+from app.config import BEIJING_TZ
 
 from fastapi import Depends, HTTPException, Security
 from fastapi.security import APIKeyHeader
@@ -57,7 +59,7 @@ async def verify_api_key(
         raise HTTPException(status_code=401, detail="Invalid API key / 无效的 API Key")
 
     # Update last_used_at
-    db_key.last_used_at = datetime.now(timezone.utc)
+    db_key.last_used_at = datetime.now(BEIJING_TZ)
     await db.flush()
 
     return {"permissions": db_key.permissions, "key_id": db_key.id, "name": db_key.name}