feat: Implement user validation and ownership checks for image, reward, and task APIs

- Added `get_validated_user_id` utility function to validate user authentication across multiple APIs.
- Updated image upload, request, and listing endpoints to ensure user ownership and proper error handling.
- Enhanced reward management endpoints to include user validation and ownership checks.
- Modified task management endpoints to enforce user authentication and ownership verification.
- Updated models to include `user_id` for images, rewards, tasks, and children to track ownership.
- Implemented frontend changes to ensure UI reflects the ownership of tasks and rewards.
- Added a new feature specification to prevent deletion of system tasks and rewards.

backend/models/task.py

@@ -7,6 +7,7 @@ class Task(BaseModel):
     points: int
     is_good: bool
     image_id: str | None = None
+    user_id: str | None = None
 
     @classmethod
     def from_dict(cls, d: dict):
@@ -15,17 +16,19 @@ class Task(BaseModel):
             points=d.get('points', 0),
             is_good=d.get('is_good', True),
             image_id=d.get('image_id'),
+            user_id=d.get('user_id'),
             id=d.get('id'),
             created_at=d.get('created_at'),
             updated_at=d.get('updated_at')
         )
-
+    
     def to_dict(self):
         base = super().to_dict()
         base.update({
             'name': self.name,
             'points': self.points,
             'is_good': self.is_good,
-            'image_id': self.image_id
+            'image_id': self.image_id,
+            'user_id': self.user_id
         })
         return base