Auto expire old broadcast messages

Since the expiry is sent as part of the message payload, we don't
need to invoke the CBC proxies (and indeed there's no way to do so
for an expired alert). In future we plan to extend this task so it
triggers the regeneration of content on gov.uk/alerts.

It's worth noting that 'finishes_at' can theoretically be None, in
which case it's unclear when the alert should expire. While alerts
from the Admin app should always have an expiry [1], we have many
in the DB that don't, so it's worth checking for this scenario.

[1]: 078ac10c8d/app/models/broadcast_message.py (L255)

app/celery/scheduled_tasks.py

@@ -4,7 +4,7 @@ from flask import current_app
 from sqlalchemy import between
 from sqlalchemy.exc import SQLAlchemyError
 
-from app import notify_celery, zendesk_client
+from app import db, notify_celery, zendesk_client
 from app.celery.broadcast_message_tasks import trigger_link_test
 from app.celery.letters_pdf_tasks import get_pdf_for_templated_letter
 from app.celery.tasks import (
@@ -48,6 +48,8 @@ from app.models import (
     JOB_STATUS_IN_PROGRESS,
     JOB_STATUS_PENDING,
     SMS_TYPE,
+    BroadcastMessage,
+    BroadcastStatusType,
     Job,
 )
 from app.notifications.process_notifications import send_notification_to_queue
@@ -304,3 +306,16 @@ def trigger_link_tests():
     if current_app.config['CBC_PROXY_ENABLED']:
         for cbc_name in current_app.config['ENABLED_CBCS']:
             trigger_link_test.apply_async(kwargs={'provider': cbc_name}, queue=QueueNames.BROADCASTS)
+
+
+@notify_celery.task(name='auto-expire-broadcast-messages')
+def auto_expire_broadcast_messages():
+    expired_broadcasts = BroadcastMessage.query.filter(
+        BroadcastMessage.finishes_at <= datetime.now(),
+        BroadcastMessage.status == BroadcastStatusType.BROADCASTING,
+    )
+
+    for broadcast in expired_broadcasts:
+        broadcast.status = BroadcastStatusType.COMPLETED
+
+    db.session.commit()