migration-reviewlisted

# Migration Review Database migrations are one of the highest-blast-radius changes a repo ships. Review every migration against these criteria before approval. Migrations are Tier 1 by default — `go-tdd-feature` or `go-tdd-bugfix` applies in addition to this review. ## Step 1: Reversibility - Is there a DOWN migration (or equivalent rollback)? - Does the DOWN migration actually undo the UP? (Common mistake: UP adds column X and backfills; DOWN drops column X but the backfilled data is gone.) - Is the DOWN migration tested, or at least executed once in a throwaway environment? - If rollback would cause data loss, is that called out explicitly? If there is no safe rollback, the PR description must say so and justify why forward-only is acceptable for this change. ## Step 2: Zero-downtime For any database with uptime requirements: - Is there any `ALTER TABLE` that locks the whole table? On Postgres, adding a column is fine; adding a `NOT NULL` column without a default on a large table is not. - Is the migration compatible with both the currently-deployed application code AND the new application code? - Does the deploy plan require: deploy schema change, let it settle, deploy app change, let it settle, cleanup? - Are indexes created concurrently (`CREATE INDEX CONCURRENTLY` on Postgres)? ## Step 3: Concurrent correctness - Are explicit `LOCK TABLE` statements scoped as tightly as possible? - Is the migration idempotent? (Re-running should be safe.) - Doe