The initial 030 used sa.inspect(op.get_bind()) for its conditional ALTERs, which
raises NoInspectionAvailable under `alembic upgrade --sql` (offline mode) —
breaking test_enum_migration_parity and any offline SQL generation. Reimplement
the same idempotent rename (text->content) + add (created_at) as a self-contained
plpgsql DO block that guards on information_schema at execution time, so it
renders offline and runs online identically. Round-trip verified.