Prod only ever runs `alembic upgrade head`; nothing checks that the migrated
schema still matches Base.metadata, and the downgrade path never runs. Add a
DB-lane test that (a) asserts no structural (table/column) drift between the ORM
models and the head schema — filtering the index/constraint/sequence noise that
compare_metadata reports for migration-only objects — and (b) exercises the head
migration's downgrade + re-upgrade so a broken downgrade is caught the sprint it
lands.
