set media_type to image/webp for get_child_image FileResponse

- automatically inferred type caused `Failed to execute 'createObjectURL' on 'URL': Overload resolution failed.` runtime error in frontend

mondey_backend/src/mondey_backend/routers/users.py

@@ -92,7 +92,7 @@ async def get_child_image(
         child = get_db_child(session, current_active_user, child_id)
         image_path = child_image_path(child_id)
         if child.has_image and image_path.exists():
-            return image_path
+            return FileResponse(image_path, media_type="image/webp")
         raise HTTPException(404)
 
     @router.put("/children-images/{child_id}")