Alembic migrations for ingestlib's internal registry DB (Postgres).

The database URL is resolved in env.py from INGESTLIB_REGISTRY_URL; this
directory has no sqlalchemy.url of its own. One table per migration, 0001→0007.

Applying the schema
-------------------
Runtime (what users run):

    ingestlib registry init

builds an Alembic Config programmatically against this directory and runs
`upgrade head`. It does NOT read alembic.ini, so it works from a plain
`pip install`. Idempotent: creates the tables on a fresh DB, upgrades a DB
that is behind head, and no-ops when already current. `ingestlib registry
status` reports the current revision.

Dev (from the repo):

    alembic upgrade head            # uses the root alembic.ini + env.py
    alembic revision -m "..."       # author a new migration

Containerized deploy (Docker Compose / Kubernetes): run `ingestlib registry
init` once as an init-container or a one-shot Job before the app starts —
migrations are never baked into an image.

Note: the compose `registry` container's init.sql runs at first boot and only
creates the read-only role `ingestlib_ro`. It does NOT create the tables — the
tables come from these migrations, applied by `registry init` (or `alembic
upgrade head`).
