Migration immutability

Applied migrations cannot be rewritten: golang-migrate skips applied versions. Treat migrations merged to main as immutable, even when deployment status is unknown. Check the creation commit's reachability from origin/main (or main when unavailable), not merely whether the file has local commit history. Merged history is a conservative boundary, not proof of deployment. Missing or shallow history is inconclusive; resolve it before declaring a file editable.

A new, unmerged migration can be revised in its feature branch unless it is known to have been deployed. Do not delete or renumber immutable migrations.

For an immutable migration, implement the requested correction in a new pair using cd server && just db-migration-new <name>. Include both up and down files and follow the generation skill to select and run the root generation command after schema/query changes. This is the normal way to complete a schema fix, not a reason to stop at proposing one.