Order alert groups by -started_at in internal API (#3240)

# What this PR does
Order alert groups in API by -started_at instead of -pk. This should
result in no visible change for most installations. In the case where
alert groups are being moved/restored between installations this allows
for the display order to be as expected without relying on insertion
order.

Note: Public API is already using -started_at instead of pk and it is
already indexed.

## Which issue(s) this PR fixes

## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Prevent additional polling on Incidents if the previous request didn't complete
   ([#3205](https://github.com/grafana/oncall/pull/3205))
 - Order results from `GET /teams` internal API endpoint by ascending name by @joeyorlando ([#3220](https://github.com/grafana/oncall/pull/3220))
+- Order alert groups internal API endpoint by descending started_at by @mderynck ([#3240](https://github.com/grafana/oncall/pull/3240))
 
 ### Fixed
 

engine/apps/api/views/alert_group.py

@@ -423,7 +423,7 @@ class AlertGroupView(
 
         # enrich alert groups with select_related and prefetch_related
         alert_group_pks = [alert_group.pk for alert_group in alert_groups]
-        queryset = AlertGroup.objects.filter(pk__in=alert_group_pks).order_by("-pk")
+        queryset = AlertGroup.objects.filter(pk__in=alert_group_pks).order_by("-started_at")
 
         queryset = self.get_serializer_class().setup_eager_loading(queryset)
         alert_groups = list(queryset)