Add account deletion scheduler and comprehensive tests

- Implemented account deletion scheduler in `account_deletion_scheduler.py` to manage user deletions based on a defined threshold.
- Added logging for deletion processes, including success and error messages.
- Created tests for deletion logic, including edge cases, retry logic, and integration tests to ensure complete deletion workflows.
- Ensured that deletion attempts are tracked and that users are marked for manual intervention after exceeding maximum attempts.
- Implemented functionality to check for interrupted deletions on application startup and retry them.

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

@@ -16,6 +16,8 @@ export interface User {
   image_id: string | null
   marked_for_deletion: boolean
   marked_for_deletion_at: string | null
+  deletion_in_progress: boolean
+  deletion_attempted_at: string | null
 }
 
 export interface Child {