Update type hints in backend/main.py by adding Dict import

backend/main.py

@@ -3,7 +3,7 @@ from fastapi import FastAPI, Depends, HTTPException
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import StreamingResponse
 from sqlalchemy.orm import Session
-from typing import Any, List, Optional
+from typing import Any, Dict, List, Optional
 import re
 import html
 import json