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)

frontend/src/lib/types.ts

@@ -107,6 +107,21 @@ export interface FileNeighbors {
   next: FileNeighbor | null;
 }
 
+export interface DownloadTask {
+  task_id: string;
+  post_id: number | null;
+  filename: string;
+  status: "pending" | "downloading" | "complete" | "error" | "cancelled";
+  progress: number;
+  downloaded: number;
+  total: number;
+  speed: number | null;
+  error: string;
+  library_j_id: string | null;
+  created_at: string;
+  updated_at: string;
+}
+
 export interface E621IqdbCandidate {
   post_id: number | null;
   score: number | null;