Type notification's 'data' field as a string

src/api/notifications.py

@@ -1,5 +1,5 @@
 from datetime import UTC, datetime
-from typing import Any, Literal, cast, final
+from typing import Literal, cast, final
 
 from beanie import Link, PydanticObjectId
 from fastapi import APIRouter, HTTPException, status
@@ -28,7 +28,7 @@ class NotificationData(BaseModel):
     user_id: PydanticObjectId
     event_type: Literal["message", "invitation"]
     message: str
-    data: Any
+    data: str
     read: bool
     created_at: datetime
     read_at: datetime | None