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/routers/commands.py

@@ -5,7 +5,9 @@ API endpoints for sending commands / messages to devices and viewing command his
 
 import logging
 import math
-from datetime import datetime, timezone
+from datetime import datetime
+
+from app.config import BEIJING_TZ
 
 from fastapi import APIRouter, Depends, HTTPException, Query, Request
 from pydantic import BaseModel, Field
@@ -129,7 +131,7 @@ async def _send_to_device(
         )
 
     command_log.status = "sent"
-    command_log.sent_at = datetime.now(timezone.utc)
+    command_log.sent_at = datetime.now(BEIJING_TZ)
     await db.flush()
     await db.refresh(command_log)
 
@@ -290,7 +292,7 @@ async def batch_send_command(request: Request, body: BatchCommandRequest, db: As
                 device.imei, body.command_type, body.command_content
             )
             cmd_log.status = "sent"
-            cmd_log.sent_at = datetime.now(timezone.utc)
+            cmd_log.sent_at = datetime.now(BEIJING_TZ)
             await db.flush()
             await db.refresh(cmd_log)
             results.append(BatchCommandResult(