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

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Co-Authored-By: HAPI <noreply@hapi.run>

app/config.py

@@ -1,9 +1,13 @@
+from datetime import timezone, timedelta
 from pathlib import Path
 from typing import Literal
 
 from pydantic import Field
 from pydantic_settings import BaseSettings
 
+# Beijing time (UTC+8)
+BEIJING_TZ = timezone(timedelta(hours=8))
+
 # Project root directory (where config.py lives → parent = app/ → parent = project root)
 _PROJECT_ROOT = Path(__file__).resolve().parent.parent
 _DEFAULT_DB_PATH = _PROJECT_ROOT / "badge_admin.db"