Why:
- Code under test (auth resolution, the two-phase upload) opens more than
one session per operation; the existing fixture only exposed one
rolled-back session.
Changes:
- Add a db_sessionmaker fixture sharing one outer transaction.
- Pin loop_scope="session" -- without it, a second async test against the
session-scoped Postgres container fails with "Event loop is closed."
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()).
