alembic/env.py previously always overwrote sqlalchemy.url from Settings().postgres.dsn, which made it impossible for a test fixture to point Alembic at a Testcontainers-managed database. Now env.py only sets it when unset, and a new integration suite runs `alembic upgrade head` against a real Postgres container per ADR-0016 (no create_all()).
Generic single-database configuration with an async dbapi.