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/urls.py

@@ -2,17 +2,14 @@ from django.urls import include, path
 from rest_framework.routers import DefaultRouter
 
 from .uploads import TempUploadViewSet
-from .views import DownloadToLibraryView, MediaItemViewSet
+from .views import ClientDownloadView, DownloadTaskViewSet, MediaItemViewSet
 
 router = DefaultRouter()
 router.register("files", MediaItemViewSet, basename="file")
 router.register("uploads", TempUploadViewSet, basename="upload")
+router.register("online/downloads", DownloadTaskViewSet, basename="download")
 
 urlpatterns = [
     path("", include(router.urls)),
-    path(
-        "online/downloads/",
-        DownloadToLibraryView.as_view(),
-        name="download_to_library",
-    ),
+    path("online/file/", ClientDownloadView.as_view(), name="client_download"),
 ]