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/config/settings.py

@@ -144,6 +144,15 @@ WATCHED_FOLDER = str(WATCHED_FOLDER)
 
 E621_BASE_URL = os.getenv("E621_BASE_URL", "https://e621.net").rstrip("/")
 USER_AGENT = os.getenv("USER_AGENT", "J621/0.1 (by J621 on e621)")
+# Hosts the client-download proxy is allowed to stream from.
+E621_MEDIA_HOSTS = [
+    host.strip()
+    for host in os.getenv(
+        "E621_MEDIA_HOSTS",
+        "static1.e621.net,static2.e621.net,static3.e621.net",
+    ).split(",")
+    if host.strip()
+]
 
 # Guest visibility: e621's anonymous default blacklist is mirrored into the
 # cache by `manage.py refresh_guest_blacklist`. This fallback is used until