Download review comments as an .xlsx workbook.

Exactly one scope selector, matching the four places the export is offered: ids=<a>,<b> — an explicit selection: one creative (review screen) or many (bulk bar) space_id=<id> — every creative in one project scope=workspace — the whole Creative Output library (projects + standalone uploads)

A GET because the workbook is a pure read and a GET is the shape the browser can hand to its own downloader. Read access is enough — exporting a change list is not a mutation, so a viewer may do it. 400 on an empty/oversized selection, 404 if any named id isn't the caller's.