test(superset): add Testcontainers setup for Apache Superset integration tests
- Add 6 fixtures to integration conftest: superset_db_url, superset_secret_key,
superset_admin_password, superset_container (init+web), superset_url,
superset_admin_headers
- Two-container architecture: init (db upgrade → create-admin → init) +
web (superset run -p 8088)
- Superset 4.1.2 pinned (6.x incompatible: no psycopg2, SUPERSET__ prefix required)
- 8 integration tests cover health, form-login auth, SupersetClient construction
- Document decision in ADR-0012 with architecture rationale, rejected alternatives,
and migration path to Superset 6.x
- Update docs/architecture.md with testing infrastructure overview
busya
79f67e1952