Backend:
- DownloadTask model + background thread runner: streams the file with
progress (%, bytes, speed) and a cancel flag, then indexes it, names it
J-<id>.<ext> and applies the e621 metadata
- DownloadTaskViewSet (create/retrieve/cancel) replaces the synchronous
endpoint; the status footer's worker counts now reflect download jobs
- Client download proxy (/api/online/file/) streams an e621 original to
the browser with Content-Disposition: attachment, restricted to the
configured e621 CDN hosts so it cannot be used as an open proxy
Frontend:
- Online detail: progress bar with percentage, transferred size, speed
and cancel while downloading; success links to the new J-ID
- New 'Download to client' button available to everyone (guests too)
