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/reward.py

@@ -8,6 +8,7 @@ class Reward(BaseModel):
     description: str
     cost: int
     image_id: str | None = None
+    user_id: str | None = None
 
     @classmethod
     def from_dict(cls, d: dict):
@@ -17,6 +18,7 @@ class Reward(BaseModel):
             description=d.get('description'),
             cost=d.get('cost', 0),
             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')
@@ -28,6 +30,7 @@ class Reward(BaseModel):
             'name': self.name,
             'description': self.description,
             'cost': self.cost,
-            'image_id': self.image_id
+            'image_id': self.image_id,
+            'user_id': self.user_id
         })
         return base