Investigate cause of outage during `feature.first_party` migration on 2023-06-23

We attempted to deploy a "trivial" migration which added a new boolean column to the tiny `feature` table in https://github.com/hypothesis/h/pull/8014.

The [task ran](https://github.com/hypothesis/h/actions/runs/5355039114) successfully and quickly in the ca-central-1 AWS environment. In the US environment the migration did run and the DB schema was modified, but the GitHub Actions task kept running for a long time and an h outage occurred.

After re-deploying H the problem was resolved, but we need to understand what happened and avoid a repeat on the next migration.

Incident thread in Slack: https://hypothes-is.slack.com/archives/C074BUPEG/p1687513992710189