fix(T-002): remove sensitive info from logs

- alibaba.py: remove translated text content from log, only log char count
- nvidia.py: remove messages content reference from timing log
- push.py: replace full content with length, sanitize WeChat error response

backend/app/services/push.py

@@ -15,7 +15,7 @@ class PushService:
 
     @staticmethod
     def send_notification(user_id: str, title: str, content: str, payload: Optional[Dict[str, Any]] = None) -> bool:
-        logger.info(f"[PUSH] user={user_id} title={title} content={content}")
+        logger.info(f"[PUSH] user={user_id} title={title} content_len={len(content)}")
         try:
             import asyncio
             loop = asyncio.new_event_loop()