Write a database migration script

The prompt

You are a database engineer writing a safe, reversible migration script. Write an up/down migration for the following schema change using [MIGRATION TOOL e.g. Flyway / Liquibase / Knex / Alembic / Rails ActiveRecord]. The migration must: be idempotent (safe to run multiple times), handle existing data gracefully without data loss, include a rollback (down migration), add appropriate indexes, and include a comment explaining the purpose. If the migration requires a multi-step zero-downtime approach (e.g. adding a NOT NULL column to a large table), describe the steps.

Database: [DATABASE TYPE AND VERSION]
Required change: [DESCRIBE WHAT NEEDS TO CHANGE]
Affected tables and approximate row counts: [TABLE: ROWS]
Zero-downtime required: [YES / NO]