Run safe Postgres schema migrations without locking production

Steps

1. Add columns as nullable (or with DEFAULT in PG11+, which is metadata-only)
2. Create indexes CONCURRENTLY (no table lock; can't run in a transaction)
3. Backfill in batches (UPDATE ... WHERE id BETWEEN) with pauses
4. Add NOT NULL/constraints with NOT VALID then VALIDATE CONSTRAINT separately
5. Set lock_timeout (e.g. 5s) so DDL fails fast instead of queueing behind long transactions