Add chore expiry notification system with scheduling and tests

- Implemented `trigger_chore_expiry.py` script to manually trigger chore expiry notifications for users via the admin API.
- Developed `chore_expiry_notification_scheduler.py` to handle the logic for sending notifications for chores expiring within the next 75 minutes.
- Created utility functions in `schedule_utils.py` to determine scheduling and deadlines for chores.
- Added comprehensive tests for the chore expiry notification system in `test_chore_expiry_notification_scheduler.py`, covering various scenarios including scheduled chores, confirmations, and user settings.

backend/main.py

@@ -26,6 +26,7 @@ from events.broadcaster import Broadcaster
 from events.sse import sse_response_for_user, send_to_user
 from api.utils import get_current_user_id
 from utils.account_deletion_scheduler import start_deletion_scheduler
+from utils.chore_expiry_notification_scheduler import start_chore_expiry_notification_scheduler
 from utils.digest_scheduler import start_digest_scheduler
 from utils.state_expiry_scheduler import start_state_expiry_scheduler
 
@@ -142,6 +143,7 @@ start_background_threads()
 start_deletion_scheduler()
 start_digest_scheduler(app)
 start_state_expiry_scheduler(app)
+start_chore_expiry_notification_scheduler(app)
 
 if __name__ == '__main__':
     app.run(debug=False, host='0.0.0.0', port=5000, threaded=True)