# What this PR does
This PR:
- modifies the `check_escalation_finished_task` celery task to:
- do stricter escalation validation based on the alert group's
escalation snapshot (see the `audit_alert_group_escalation` method in
`engine/apps/alerts/tasks/check_escalation_finished.py` for the
validation logic)
- use a read-only database for querying alert-groups if one is
configured, otherwise use the "default" one
- ping a configurable heartbeat (new env var
`ALERT_GROUP_ESCALATION_AUDITOR_CELERY_TASK_HEARTBEAT_URL` added)
- increase the task frequency from every 10 to every 13 minutes (this
can be configured via an env variable)
- adds public documentation on how to configure this auditor task
- modifies the local celery startup command to properly take into
consideration all celery related env vars (similar to the ones we use in
`engine/celery_with_exporter.sh`; this made it easier to enable `celery
beat` locally for testing)
- removes the following code:
- removes references to `AlertGroup.estimate_escalation_finish_time` and
marks the model field as deprecated using the [`django-deprecate-fields`
library](https://pypi.org/project/django-deprecate-fields/). This field
was only used for the previous version of this validation task
- `EscalationSnapshotMixin.calculate_eta_for_finish_escalation` was only
used to calculate the value for
`AlertGroup.estimate_escalation_finish_time`
- `calculate_escalation_finish_time` celery task
## Which issue(s) this PR fixes
https://github.com/grafana/oncall-private/issues/1558
## Checklist
- [x] Tests updated
- [x] Documentation added
- [x] `CHANGELOG.md` updated
