Schedule Price Change

Insert a new effective-date row.

expected_row_version is the row_version the UI saw when it loaded the form. If a concurrent save bumped it on the same (dataset_id, cut, access_type) since then, the service raises StaleStateError and the UI prompts a reload.

For new SKUs that don't have any prior row, pass expected_row_version=0 so the service can distinguish first insert from a stale concurrent overwrite.