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/protocol/builder.py

@@ -8,7 +8,9 @@ from __future__ import annotations
 
 import struct
 import time
-from datetime import datetime, timezone
+from datetime import datetime
+
+from app.config import BEIJING_TZ
 from typing import Optional
 
 from .constants import (
@@ -141,9 +143,9 @@ class PacketBuilder:
         """
         Build a Time Sync 2 response (0x8A).
 
-        Returns the current UTC time as YY MM DD HH MM SS (6 bytes).
+        Returns the current Beijing time as YY MM DD HH MM SS (6 bytes).
         """
-        now = datetime.now(timezone.utc)
+        now = datetime.now(BEIJING_TZ)
         info = struct.pack(
             "BBBBBB",
             now.year - 2000,