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/serializers.py

@@ -4,7 +4,7 @@ from pathlib import Path
 from django.core import signing
 from rest_framework import serializers
 
-from .models import MediaItem, MediaLocation, TempUpload
+from .models import MediaItem, MediaLocation, TempUpload, DownloadTask
 from .services import MEDIA_FILE_SALT, UPLOAD_FILE_SALT, VIDEO_EXTENSIONS
 
 
@@ -169,3 +169,29 @@ class TempUploadSerializer(serializers.ModelSerializer):
             salt=MEDIA_FILE_SALT,
         )
         return f"/api/files/J-{item.id}/{action}/?sig={signature}"
+
+
+class DownloadTaskSerializer(serializers.ModelSerializer):
+    task_id = serializers.UUIDField(source="id", read_only=True)
+    library_j_id = serializers.SerializerMethodField()
+
+    class Meta:
+        model = DownloadTask
+        fields = [
+            "task_id",
+            "post_id",
+            "filename",
+            "status",
+            "progress",
+            "downloaded",
+            "total",
+            "speed",
+            "error",
+            "library_j_id",
+            "created_at",
+            "updated_at",
+        ]
+        read_only_fields = fields
+
+    def get_library_j_id(self, obj):
+        return f"J-{obj.library_item_id}" if obj.library_item_id else None