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/parser.py

@@ -9,7 +9,9 @@ start markers.
 from __future__ import annotations
 
 import struct
-from datetime import datetime, timezone
+from datetime import datetime
+
+from app.config import BEIJING_TZ
 from typing import Any, Dict, List, Tuple
 
 from .constants import (
@@ -226,7 +228,7 @@ class PacketParser:
         yy, mo, dd, hh, mi, ss = struct.unpack_from("BBBBBB", data, offset)
         year = 2000 + yy
         try:
-            dt = datetime(year, mo, dd, hh, mi, ss, tzinfo=timezone.utc)
+            dt = datetime(year, mo, dd, hh, mi, ss, tzinfo=BEIJING_TZ)
         except ValueError:
             dt = None
         return {