fix(activity): record external.* TaskRun.queue as download, not default (#883)

celery_signals._queue_for is a hand-maintained mirror of task_routes that
stamps TaskRun.queue in the prerun signal. It was missing the
backend.app.tasks.external. prefix, so external fetches recorded
queue='default' even though celery routes external.* → download and runs
them on the download worker. The dashboard's per-queue filters and the
per-queue recovery-sweep threshold therefore missed them — the same
'queue column lies default' gap the 2026-06-02 audit fixed for
backup/admin/library_audit.

Map external.* → download in _queue_for. Composes with the fetch_external_link
task-name sweep override (#883), which wins by precedence regardless of the
recorded queue. Pinned test asserts the mirror agrees with the actual route.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_celery_routing.py

@@ -18,3 +18,21 @@ def test_long_one_shots_route_to_maintenance_long():
 def test_quick_maintenance_stays_on_maintenance():
     routes = celery.conf.task_routes
     assert routes["backend.app.tasks.maintenance.*"]["queue"] == "maintenance"
+
+
+def test_queue_for_mirrors_external_to_download():
+    """celery_signals._queue_for is a hand-maintained mirror of task_routes
+    that stamps TaskRun.queue. external.* routes to the download lane, so the
+    mirror must agree — else TaskRun.queue lies 'default' for external fetches
+    and per-queue dashboard filters / threshold overrides miss them
+    (operator-flagged 2026-06-17)."""
+    from backend.app.celery_signals import _queue_for
+
+    class _T:
+        name = "backend.app.tasks.external.fetch_external_link"
+
+    assert _queue_for(_T()) == "download"
+    assert (
+        celery.conf.task_routes["backend.app.tasks.external.*"]["queue"]
+        == "download"
+    )