test(migrations): restore PostgreSQL full-chain idempotence coverage

[bsbds]

Problem

tests/migrations/test_migration_integration.py cannot currently run the complete PostgreSQL Alembic history against a schema pre-created from current SQLAlchemy metadata. The owner-aware UserOAuth migration test therefore stamps the database to its immediate parent before running the target revision.

This avoids unrelated failures in older revisions, but it no longer verifies full-chain re-run and Alembic version stability for this path.

A likely incompatibility is the conditional foreign-key creation in c7dfa28cc67a_add_user_oauth_table.py. Other historical revisions may have similar assumptions about an empty or revision-matched schema.

Desired outcome

• Identify the historical revisions that fail when the current metadata schema already exists.
• Define whether pre-created-current-metadata plus full-history migration is a supported contract.
• If supported, make the affected revisions idempotent without changing fresh-database behavior.
• Restore a PostgreSQL integration test that runs the full migration chain and verifies a second run is stable.
• Document any intentionally unsupported initialization path if the contract is rejected.

Context

Tracked from review feedback on #1529. This is pre-existing migration-chain maintenance and is intentionally kept outside that PR.