Ecto: migrations are ordered, immutable, safe, reversible, and back every schema change.
Option since: (a ref such as "origin/main") makes only migrations present on that ref
immutable, so unmerged ones can still be edited.
Rules
migrations_ordered— added migrations are newer than every existing onemigrations_immutable— committed migrations are never editedschema_changes_migrated— schema columns are added by a migration in the same commitindexes_concurrent— indexes are created concurrentlymigrations_reversible— added migrations can be rolled back