Bundles a selection of style documents (by fileId) into a single ZIP archive and returns a signed download URL. Scoped to the caller's company; ids that do not belong to this style or company are rejected all-or-nothing with a 404 and details.fileIds listing the offending ids. A non-existent style returns style_not_found (404). An empty selection is rejected with a 400 by the route schema (min length 1) and again by the service as empty_file_ids.