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/models.py

@@ -115,3 +115,57 @@ class TempUpload(models.Model):
 
     def __str__(self):
         return f"{self.original_filename} ({self.status})"
+
+
+class DownloadTask(models.Model):
+    """A background 'Download to Library' job with progress tracking."""
+
+    STATUS_PENDING = "pending"
+    STATUS_DOWNLOADING = "downloading"
+    STATUS_COMPLETE = "complete"
+    STATUS_ERROR = "error"
+    STATUS_CANCELLED = "cancelled"
+    STATUS_CHOICES = [
+        (STATUS_PENDING, "Pending"),
+        (STATUS_DOWNLOADING, "Downloading"),
+        (STATUS_COMPLETE, "Complete"),
+        (STATUS_ERROR, "Error"),
+        (STATUS_CANCELLED, "Cancelled"),
+    ]
+
+    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
+    user = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        null=True,
+        blank=True,
+        on_delete=models.SET_NULL,
+        related_name="download_tasks",
+    )
+    post_id = models.IntegerField(null=True, blank=True)
+    url = models.CharField(max_length=1000)
+    filename = models.CharField(max_length=255, blank=True, default="")
+    e621_data = models.JSONField(null=True, blank=True)
+    status = models.CharField(
+        max_length=20, choices=STATUS_CHOICES, default=STATUS_PENDING
+    )
+    progress = models.IntegerField(default=0)
+    downloaded = models.BigIntegerField(default=0)
+    total = models.BigIntegerField(default=0)
+    speed = models.FloatField(null=True, blank=True)
+    error = models.TextField(blank=True, default="")
+    cancelled = models.BooleanField(default=False)
+    library_item = models.ForeignKey(
+        MediaItem,
+        null=True,
+        blank=True,
+        on_delete=models.SET_NULL,
+        related_name="download_tasks",
+    )
+    created_at = models.DateTimeField(auto_now_add=True)
+    updated_at = models.DateTimeField(auto_now=True)
+
+    class Meta:
+        ordering = ["-created_at"]
+
+    def __str__(self):
+        return f"Download {self.id} ({self.status})"