Schema migration locks entire table

Postgres 14. A schema migration (ALTER TABLE accounts ADD COLUMN ...) was deployed at 16:30 and the deploy 'hung'. Within 2 minutes, p99 on the entire accounts-touching API spiked and pg_stat_activity filled with sessions in wait_event = relation / Lock. The ALTER itself is shown as active but waiting. There's also a long-running analytics SELECT on accounts that started at 16:25 and a steady stream of normal app queries. Explain the lock cascade, how you triage with the lock views, the safe mitigation, and how to ship such migrations without an outage.