fix: SqliteSaver context manager error

graph.py

@@ -12,13 +12,15 @@ from langchain_core.runnables import RunnableConfig
 from langchain_openrouter import ChatOpenRouter
 from typing import TypedDict, Annotated
 
+import sqlite3
 from langgraph.checkpoint.sqlite import SqliteSaver
 from config import OPENROUTER_API_KEY, DB_PATH
 import prompts
 
 
 # --- Checkpointer (SQLite — no external DB needed) ---
-checkpointer = SqliteSaver.from_conn_string(DB_PATH)
+_conn = sqlite3.connect(DB_PATH, check_same_thread=False)
+checkpointer = SqliteSaver(conn=_conn)
 
 
 # --- Model config ---