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."
Testcontainers reports the container host as `localhost`, which resolves
to ::1 before 127.0.0.1 on this machine. Docker publishes the mapped port
on IPv4 only, and the IPv6 SYN is dropped rather than refused, so asyncpg
blocked on the first address until its connect timeout instead of falling
back to the second -- the migration fixture hung rather than failing.
127.0.0.1 connects in 0.07s where localhost timed out at 15s; the
integration suite now runs in 3.6s, inside the 10s pytest-timeout.
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()).
