The drift check dropped every diff except table/column presence, so a column
type or nullability change would slip through. Add modify_type/modify_nullable
to the ops it fails on. That immediately surfaced a real pre-existing drift:
notifications.created_at was created NULLABLE by migration 0017 but the model
has always declared it NOT NULL (Mapped[datetime] + server_default=func.now()).
Migration 0066 enforces NOT NULL to match the model — safe, since the
server_default has filled every row since day one.
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.
peter