fix(chat/wechat): messages order by created_at,id (sub-second DATETIME(3)); container+MySQL TZ->Asia/Shanghai; correct UTC timestamps to CST; rebuild WeXin Bot history

backend/python/app/api/im_bot_user.py

@@ -17,7 +17,7 @@ import json
 import random
 import threading
 import time
-from datetime import datetime
+from datetime import datetime, timedelta
 from typing import Optional
 
 import requests
@@ -310,8 +310,11 @@ def _mirror_to_ai_chat(user_id: str, user_content: str, assistant_content: str):
     try:
         db = SessionLocal()
         cid = _get_or_create_wechat_conv(db, user_id)
-        _append_ai_message(db, cid, "user", user_content)
-        _append_ai_message(db, cid, "assistant", assistant_content)
+        now = datetime.now()
+        _append_ai_message(db, cid, "user", user_content, created=now)
+        # assistant 比 user 晚 50ms，保证排序严格递增，避免同秒时 ORDER BY 乱序
+        _append_ai_message(db, cid, "assistant", assistant_content,
+                            created=now + timedelta(milliseconds=50))
         _execute_raw(db, text("UPDATE conversations SET updated_at=:u WHERE id=:id"),
                      {"u": datetime.now(), "id": cid})
     except Exception: