Async Download to Library with progress; Download to client

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)

backend/apps/library/services.py

@@ -189,17 +189,32 @@ def serve_file(request, path, download=False):
     return response
 
 
-def download_file(url, destination):
+class DownloadCancelled(Exception):
+    """Raised when a streamed download is cancelled by the user."""
+
+
+def download_file(
+    url, destination, progress_callback=None, should_cancel=None
+):
     """Stream a remote file into destination (used by Download to Library)."""
     import requests
 
     headers = {"User-Agent": settings.USER_AGENT}
     with requests.get(url, headers=headers, stream=True, timeout=120) as response:
         response.raise_for_status()
+        total = int(response.headers.get("content-length") or 0)
+        downloaded = 0
         with open(destination, "wb") as handle:
             for chunk in response.iter_content(chunk_size=CHUNK_SIZE):
+                if should_cancel is not None and should_cancel():
+                    raise DownloadCancelled("download cancelled")
                 if chunk:
                     handle.write(chunk)
+                    downloaded += len(chunk)
+                    if progress_callback is not None:
+                        progress_callback(downloaded, total)
+        if progress_callback is not None:
+            progress_callback(downloaded, total or downloaded)
 
 
 E621_DESCRIPTION_LIMIT = 20000