修复 AsyncPostgresSaver 导入路径，直接从 langgraph 导入

backend/app/backend.py

@@ -14,7 +14,7 @@ from fastapi import FastAPI, HTTPException, WebSocket, WebSocketDisconnect, Depe
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import StreamingResponse
 from pydantic import BaseModel
-from app.main_graph.checkpoint.postgres.aio import AsyncPostgresSaver
+from langgraph.checkpoint.postgres.aio import AsyncPostgresSaver
 from .agent.service import AIAgentService
 from .agent.history import ThreadHistoryService
 from app.core.human_review import (