feat(events): harden registration checkout lifecycle

apps/events/tasks.py

@@ -5,12 +5,19 @@ import logging
 from celery import shared_task
 
 from apps.events.models import Registration
+from apps.events.checkout import expire_stale_registration_checkouts
 from apps.notifications.services import notify_user
 from apps.users.tasks import send_critical_sms
 
 logger = logging.getLogger(__name__)
 
 
+@shared_task(name="apps.events.expire_stale_registration_checkouts")
+def expire_stale_registration_checkouts_task():
+    """Release seats held by new checkout attempts that reached their deadline."""
+    return expire_stale_registration_checkouts()
+
+
 @shared_task(bind=True, max_retries=3)
 def send_registration_confirmation_notification(self, registration_pk: str):
     try: