Rewrite FactStatus updates to be an upsert

This is consistent with the way we do billing updates [1] and is a
bit less clunky. Functionally it should be the same - note that the
tests already cover the "overwriting" behaviour if a row exists.

[1]: 9ce6d2fe92/app/dao/fact_billing_dao.py (L522)

app/dao/fact_notification_status_dao.py

@@ -65,15 +65,9 @@ def fetch_status_data_for_service_and_day(process_day, service_id, notification_
 @autocommit
 def update_fact_notification_status(new_status_rows, process_day, notification_type, service_id):
     table = FactNotificationStatus.__table__
-    FactNotificationStatus.query.filter(
-        FactNotificationStatus.bst_date == process_day,
-        FactNotificationStatus.notification_type == notification_type,
-        FactNotificationStatus.service_id == service_id,
-    ).delete()
 
     for row in new_status_rows:
-        db.session.connection().execute(
-            insert(table).values(
+        stmt = insert(table).values(
             bst_date=process_day,
             template_id=row.template_id,
             service_id=service_id,
@@ -83,8 +77,17 @@ def update_fact_notification_status(new_status_rows, process_day, notification_t
             notification_status=row.status,
             notification_count=row.notification_count,
         )
+
+        stmt = stmt.on_conflict_do_update(
+            constraint="ft_notification_status_pkey",
+            set_={
+                FactNotificationStatus.notification_count: stmt.excluded.notification_count,
+                FactNotificationStatus.updated_at: datetime.utcnow()
+            }
         )
 
+        db.session.connection().execute(stmt)
+
 
 def fetch_notification_status_for_service_by_month(start_date, end_date, service_id):
     return db.session.query(