- Add e1f2a3b4c5d6 migration: create dataset_review_sessions with FK
ondelete='CASCADE' (matching the model). Previously created at runtime
by init_db() → create_all().
- Update f0e9d8c7b6a5: change down_revision from a5b6c7d8e9f0 to
e1f2a3b4c5d6 so the table exists when this migration runs.
- All other create_all()-only tables (llm_providers, roles,
validation_policies, llm_validation_results) already have guards
(_table_exists()) in their respective migrations.