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>

backend/app/celery_signals.py

@@ -69,7 +69,15 @@ def _queue_for(task) -> str:
         return "ml"
     if name.startswith("backend.app.tasks.thumbnail."):
         return "thumbnail"
-    if name.startswith("backend.app.tasks.download."):
+    if name.startswith((
+        "backend.app.tasks.download.",
+        # External file-host fetches share the download lane (celery_app
+        # routes external.* → download). Mirror it here or TaskRun.queue
+        # lies 'default' for them, so per-queue dashboard filters and the
+        # per-queue threshold override miss them — the same gap the
+        # 2026-06-02 audit fixed for backup/admin/library_audit.
+        "backend.app.tasks.external.",
+    )):
         return "download"
     if name.startswith("backend.app.tasks.scan."):
         return "scan"