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/migrations/0005_downloadtask.py

@@ -0,0 +1,41 @@
+# Generated by Django 6.1.1 on 2026-09-17 17:03
+
+import django.db.models.deletion
+import uuid
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('library', '0004_tempupload'),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='DownloadTask',
+            fields=[
+                ('id', models.UUIDField(default=uuid.uuid4, editable=False, primary_key=True, serialize=False)),
+                ('post_id', models.IntegerField(blank=True, null=True)),
+                ('url', models.CharField(max_length=1000)),
+                ('filename', models.CharField(blank=True, default='', max_length=255)),
+                ('e621_data', models.JSONField(blank=True, null=True)),
+                ('status', models.CharField(choices=[('pending', 'Pending'), ('downloading', 'Downloading'), ('complete', 'Complete'), ('error', 'Error'), ('cancelled', 'Cancelled')], default='pending', max_length=20)),
+                ('progress', models.IntegerField(default=0)),
+                ('downloaded', models.BigIntegerField(default=0)),
+                ('total', models.BigIntegerField(default=0)),
+                ('speed', models.FloatField(blank=True, null=True)),
+                ('error', models.TextField(blank=True, default='')),
+                ('cancelled', models.BooleanField(default=False)),
+                ('created_at', models.DateTimeField(auto_now_add=True)),
+                ('updated_at', models.DateTimeField(auto_now=True)),
+                ('library_item', models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='download_tasks', to='library.mediaitem')),
+                ('user', models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='download_tasks', to=settings.AUTH_USER_MODEL)),
+            ],
+            options={
+                'ordering': ['-created_at'],
+            },
+        ),
+    ]