Zip downloads don't announce their size

When downloading an album or a selection as a zip, Chrome shows no total size, so there's no progress or time remaining. POST /api/download/archive responds without a Content-Length, while single originals have one.

The zips are stored without compression, so the size can be worked out up front from the file sizes plus zip's fixed overhead per file. /api/download/info already returns the sizes.

Could the archive endpoint send Content-Length?