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/components/child/ChildView.vue

@@ -48,6 +48,7 @@ function handleRewardTriggered(event: Event) {
   const payload = event.payload as ChildRewardTriggeredEventPayload
   if (child.value && payload.child_id == child.value.id) {
     child.value.points = payload.points
+    childRewardListRef.value?.refresh()
   }
 }