Add job cancellation to the stats dashboard

- Active jobs on /stats get a cancel button wired to the existing
  download/match cancel endpoints, showing "cancelling..." and an inline
  error when the task already finished.
- Cancelling now sets the status immediately, so a task whose runner died
  in a restart stops showing as "downloading".
- Download streams use a bounded read timeout (10 s connect / 60 s read):
  a stalled socket fails within a minute (previously it could block
  forever), and a task cancelled while stalled is marked cancelled rather
  than error.
- The stats job list reaps stale download/match tasks, so phantom jobs
  never appear on the dashboard.

backend/apps/library/downloads.py

@@ -144,13 +144,25 @@ def run_download_task(task_id):
         )
     except Exception as exc:  # noqa: BLE001 - report background failures
         destination.unlink(missing_ok=True)
-        logger.exception("Download task %s failed", task_id)
-        DownloadTask.objects.filter(id=task_id).update(
-            status=DownloadTask.STATUS_ERROR,
-            error=str(exc),
-            speed=None,
-            updated_at=timezone.now(),
-        )
+        if DownloadTask.objects.filter(id=task_id, cancelled=True).exists():
+            # Cancel was requested while the socket was stalled; the read
+            # timeout is what breaks the worker out of it.
+            DownloadTask.objects.filter(id=task_id).update(
+                status=DownloadTask.STATUS_CANCELLED,
+                progress=0,
+                downloaded=0,
+                speed=None,
+                error="",
+                updated_at=timezone.now(),
+            )
+        else:
+            logger.exception("Download task %s failed", task_id)
+            DownloadTask.objects.filter(id=task_id).update(
+                status=DownloadTask.STATUS_ERROR,
+                error=str(exc),
+                speed=None,
+                updated_at=timezone.now(),
+            )
     finally:
         # Background threads hold their own DB connection; release it so
         # Gunicorn workers do not leak connections when threads finish.

backend/apps/library/services.py

@@ -250,13 +250,24 @@ class DownloadCancelled(Exception):
 
 
 def download_file(
-    url, destination, progress_callback=None, should_cancel=None
+    url,
+    destination,
+    progress_callback=None,
+    should_cancel=None,
+    read_timeout=60,
 ):
-    """Stream a remote file into destination (used by Download to Library)."""
+    """Stream a remote file into destination (used by Download to Library).
+
+    The read timeout bounds how long a stalled connection can block the
+    worker: without it a hung socket would keep a job "downloading" forever
+    and the cancel flag could never be observed.
+    """
     import requests
 
     headers = {"User-Agent": settings.USER_AGENT}
-    with requests.get(url, headers=headers, stream=True, timeout=120) as response:
+    with requests.get(
+        url, headers=headers, stream=True, timeout=(10, read_timeout)
+    ) as response:
         response.raise_for_status()
         total = int(response.headers.get("content-length") or 0)
         downloaded = 0

backend/apps/library/views.py

@@ -465,7 +465,10 @@ class DownloadTaskViewSet(
                 status=status.HTTP_400_BAD_REQUEST,
             )
         DownloadTask.objects.filter(id=task.id).update(
-            cancelled=True, updated_at=timezone.now()
+            cancelled=True,
+            status=DownloadTask.STATUS_CANCELLED,
+            speed=None,
+            updated_at=timezone.now(),
         )
         return Response({"success": True})
 
@@ -508,7 +511,9 @@ class MatchTaskViewSet(
                 status=status.HTTP_400_BAD_REQUEST,
             )
         MatchTask.objects.filter(id=task.id).update(
-            cancelled=True, updated_at=timezone.now()
+            cancelled=True,
+            status=MatchTask.STATUS_CANCELLED,
+            updated_at=timezone.now(),
         )
         return Response({"success": True})