There is no magic bullet to database migration unless your app is deployed with the database itself and can atomically switch to new version as a DDL change is committed, or you take down the whole ship in maintenance mode and bring it back up once app + DB is migrated.

The only way to do it without downtime is to make the app forward compatible before making DB changes, and make it backward compatible before undoing the DB changes.

There will always be a short period where the app version and DB version will be out of sync.