Migration hangs waiting for lock

Every deploy runs migrations as a gated pre-step. Today's 15:00 deploy hangs in the migration step: the migration is a quick ADD COLUMN ... DEFAULT now() that should take a second, but the deploy pipeline is stuck and times out after 10 minutes, blocking the release of an urgent hotfix. Dashboards: the app is otherwise healthy (the migration's ALTER hasn't acquired its lock so it isn't blocking live queries yet, it's waiting), pg_stat_activity shows the ALTER in lock not available/waiting, and the migration tool is configured with a lock_timeout of 0 (infinite) so it just waits. Ahead of it in the lock queue: a long-running analytics transaction that's been open for 40 minutes. New queries are starting to queue behind the ALTER. Triage and mitigate.