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/pending_reward.py

@@ -5,6 +5,7 @@ from models.base import BaseModel
 class PendingReward(BaseModel):
     child_id: str
     reward_id: str
+    user_id: str    
     status: str = "pending"  # pending, approved, rejected
 
     @classmethod
@@ -13,6 +14,7 @@ class PendingReward(BaseModel):
             child_id=d.get('child_id'),
             reward_id=d.get('reward_id'),
             status=d.get('status', 'pending'),
+            user_id=d.get('user_id'),
             id=d.get('id'),
             created_at=d.get('created_at'),
             updated_at=d.get('updated_at')
@@ -23,6 +25,7 @@ class PendingReward(BaseModel):
         base.update({
             'child_id': self.child_id,
             'reward_id': self.reward_id,
-            'status': self.status
+            'status': self.status,
+            'user_id': self.user_id
         })
         return base