ThreatLens Alembic migrations (single SQLite database).

Shipped inside the package so an installed copy can create and upgrade
databases:

    python -m threatlens.db.migrate db/threatlens.db

From a repo checkout the alembic CLI works too, using the root alembic.ini:

    alembic upgrade head
    alembic revision --autogenerate -m "describe the change"
