Built-in Baton.ResultStore backend — stores large step results in the
workflow_artifacts table.
This is the default backend, keeping Baton's "no extra infrastructure beyond
Oban and Postgres" guarantee. Bytes live in the data column; put/3 upserts
on the unique key, so a retried step overwrites its previous artifact rather
than leaving an orphan. Cleanup is handled by Baton.Retention, which sweeps
workflow_artifacts with the same dead-job / by-workflow logic it applies to
the other Baton tables.