The constant migrator.
All SQL!
Have a migrations table. when new migrations are found, simply insert them into the database, with the contents of the migration up/down in text fields. Have a trigger run when new rows are inserted to migrate appropriately. Allow foa lock of migration version via some other table/field, with an update trigger to migrate as needed.
Keep a log of all actions, period, even when rolling up/down, and log the output and any errors.
Push to pubsub queues for output, status, etc. for interface(s). Expose via web sockets for API.
albatross_versions table, same signature, same everything but migrates first with 'Upgrading...' message/log event.