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.

frontend/vue-app/src/stores/auth.ts

@@ -24,6 +24,8 @@ export function loginUser() {
 
 export function logoutUser() {
   isUserLoggedIn.value = false
+  currentUserId.value = ''
+  logoutParent()
 }
 
 export async function checkAuth() {