Alembic migrations for foilstack.

The database URL is read from the environment via `foilstack.config`, not from
alembic.ini — see env.py.

    uv run alembic upgrade head                 # apply
    uv run alembic revision --autogenerate -m "what changed"
    uv run alembic downgrade -1                 # step back

The container runs `alembic upgrade head` before starting the web process, so a
deploy applies its own migrations.
