Database migrations for FoundryCLI.

These live inside the package so that an installed copy of FoundryCLI
can still bring a database up to date. Alembic loads them by path at
runtime - `foundry/core/bootstrap.py` points the Alembic config at
this directory.

Each file in `versions/` is one revision, applied in order. The
application stamps the revision table when it creates the schema
itself, so a fresh database and a migrated one end up in the same
state.
