Safe Postgres migrations

Description

Write, plan, and review safe PostgreSQL schema migrations for live systems (Supabase, raw SQL, EF Core, Prisma, Drizzle) — expand/contract for zero-downtime changes, lock-aware DDL (lock_timeout, CREATE INDEX CONCURRENTLY, NOT VALID constraints then VALIDATE), adding columns and defaults, renames and type changes, batched backfills, enum changes, RLS and grants for new objects, transaction behavior of migration tools, testing on production-like data, and rollback/forward-fix plans. Use whenever the user writes or reviews a migration, alters a table in production, adds an index or constraint to a large table, renames or drops a column, or asks why a deploy locked the database.