set letter notifications to pending while notify-ftp does its stuff

this means that if the task is accidentally ran twice (eg we autoscale
notify-celery-worker-beat to 2), it won't send letters twice.

Additionally, update some function names and config variables to make
it clear that they are referring to letter jobs, rather than all letter
content

app/dao/notifications_dao.py

@@ -460,7 +460,22 @@ def is_delivery_slow_for_provider(
 
 @statsd(namespace="dao")
 @transactional
-def dao_update_notifications_sent_to_dvla(job_id, provider):
+def dao_update_notifications_for_job_to_sent_to_dvla(job_id, provider):
+    now = datetime.utcnow()
+    updated_count = db.session.query(
+        Notification).filter(Notification.job_id == job_id).update(
+        {'status': NOTIFICATION_SENDING, "sent_by": provider, "sent_at": now})
+
+    db.session.query(
+        NotificationHistory).filter(NotificationHistory.job_id == job_id).update(
+        {'status': NOTIFICATION_SENDING, "sent_by": provider, "sent_at": now, "updated_at": now})
+
+    return updated_count
+
+
+@statsd(namespace="dao")
+@transactional
+def dao_update_notifications_by_reference_sent_to_dvla(notification_references, provider):
     now = datetime.utcnow()
     updated_count = db.session.query(
         Notification).filter(Notification.job_id == job_id).update(
@@ -555,3 +570,29 @@ def dao_get_total_notifications_sent_per_day_for_performance_platform(start_date
         NotificationHistory.key_type != KEY_TYPE_TEST,
         NotificationHistory.notification_type != LETTER_TYPE
     ).one()
+
+
+def dao_set_created_live_letter_api_notifications_to_pending():
+    """
+    Sets all past scheduled jobs to pending, and then returns them for further processing.
+
+    this is used in the run_scheduled_jobs task, so we put a FOR UPDATE lock on the job table for the duration of
+    the transaction so that if the task is run more than once concurrently, one task will block the other select
+    from completing until it commits.
+    """
+    return db.session.query(
+        Notification.id
+    ).filter(
+        Notification.notification_type == LETTER_TYPE,
+        Notification.status == NOTIFICATION_CREATED,
+        Notification.key_type == KEY_TYPE_NORMAL,
+    ).with_for_update(
+    ).all()
+
+    for notification in notifications:
+        notification.notification_status = NOTIFICATION_PENDING
+
+    db.session.add_all(notifications)
+    db.session.commit()
+
+    return notifications