feat: Implement account deletion (mark for removal) feature

- Added `marked_for_deletion` and `marked_for_deletion_at` fields to User model (Python and TypeScript) with serialization updates
- Created POST /api/user/mark-for-deletion endpoint with JWT auth, error handling, and SSE event trigger
- Blocked login and password reset for marked users; added new error codes ACCOUNT_MARKED_FOR_DELETION and ALREADY_MARKED
- Updated UserProfile.vue with "Delete My Account" button, confirmation modal (email input), loading state, success/error modals, and sign-out/redirect logic
- Synced error codes and model fields between backend and frontend
- Added and updated backend and frontend tests to cover all flows and edge cases
- All Acceptance Criteria from the spec are complete and verified

frontend/vue-app/src/common/models.ts

@@ -8,6 +8,16 @@ export interface Task {
 }
 export const TASK_FIELDS = ['id', 'name', 'points', 'is_good', 'image_id'] as const
 
+export interface User {
+  id: string
+  first_name: string
+  last_name: string
+  email: string
+  image_id: string | null
+  marked_for_deletion: boolean
+  marked_for_deletion_at: string | null
+}
+
 export interface Child {
   id: string
   name: string