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.

.gitignore

@@ -4,3 +4,4 @@ backend/test_data/db/pending_rewards.json
 backend/test_data/db/rewards.json
 backend/test_data/db/tasks.json
 backend/test_data/db/users.json
+logs/account_deletion.log