fix: created_at type in MessageOut

src/tracker/api/chats.py

@@ -1,6 +1,7 @@
 """Chats and messages API."""
 
 import uuid
+from datetime import datetime
 
 from fastapi import APIRouter, Depends, HTTPException, Query
 from pydantic import BaseModel
@@ -37,7 +38,7 @@ class MessageOut(BaseModel):
     sender_id: uuid.UUID | None
     sender_name: str | None
     content: str
-    created_at: str
+    created_at: datetime
 
     model_config = {"from_attributes": True}