ALTER TABLE lock queue

A routine migration to add a column ran during a low-traffic window and 'hung'. Within a minute, the entire `users` table became effectively unavailable — every read and write to it is timing out, app error rate spiked to 100% on those endpoints, and the migration's `ALTER TABLE` is still `active`. `pg_locks` shows the ALTER waiting on an `AccessExclusiveLock`, and behind it a long queue of normally-fast SELECTs and UPDATEs all waiting. There's one old transaction at the front holding an `AccessShareLock` on `users` that's been open for 40 minutes. Triage and mitigate.