Delete a stack (ungroup its frames)

Dissolves the stack: the grouping is removed and every member frame returns to loose, individual display. The photos themselves are untouched — nothing is trashed or deleted from the library; like remove_assets_from_album, this only removes an organizational grouping. Use trash_assets to soft-delete the underlying assets. If a concurrent mutation adds a frame mid-delete, returns 409 and nothing is changed; retry the request.