fix(downloads): rewire MaintenanceMenu to the downloads pipeline

The maintenance dropdown in Subscriptions → Downloads was wired to the
filesystem-import pipeline (POST /api/import/retry-failed +
POST /api/import/clear-stuck) — the subtitles even said so ("Re-enqueue
every failed import task"), but it was contextually misplaced. From the
Downloads view "Retry failed" queued nothing the operator could see
because the action operated on import_task rows, not download_event
rows. Import-pipeline maintenance is already reachable from Settings →
Imports (ImportTaskList.vue), so removing the import wiring loses
nothing.

Rewired:
- "Retry failed" → bulk-retries the failing-sources list, same loop as
  FailingSourcesCard's RETRY ALL (sourcesStore.checkNow per source).
  Subtitle now matches: "Re-queue every currently failing source".
- "Force recovery sweep" → triggers recover_stalled_download_events on
  demand via a new POST /api/downloads/recover-stalled endpoint. The
  sweep also runs every 5 min on Beat; this is the manual fallback so
  the operator doesn't have to wait for the next tick to clear newly
  stranded events.

MaintenanceMenu is now stateless — emits retry-failed and recover-
stalled. DownloadsTab owns the handlers (reuses the existing
onRetryAll; new onRecoverStalled with a delayed refresh so swept rows
land in the failing rollup).

Operator-flagged 2026-05-29 — "the retry failed button in the
maintenance dropdown doesn't appear to queue anything but manual
requeues works."

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_api_downloads.py

@@ -134,3 +134,23 @@ async def test_activity_returns_fixed_bucket_array(client, seed):
 async def test_activity_rejects_bogus_hours(client):
     resp = await client.get("/api/downloads/activity?hours=bogus")
     assert resp.status_code == 400
+
+
+@pytest.mark.asyncio
+async def test_recover_stalled_dispatches_task(client, monkeypatch):
+    """POST /api/downloads/recover-stalled returns 202 and dispatches the
+    recover_stalled_download_events task. The task body itself is exercised
+    in test_maintenance.py — this just covers the API → Celery hand-off."""
+    from backend.app.tasks import maintenance
+
+    called: list[tuple] = []
+    monkeypatch.setattr(
+        maintenance.recover_stalled_download_events, "delay",
+        lambda *a, **kw: called.append((a, kw)),
+    )
+
+    resp = await client.post("/api/downloads/recover-stalled")
+    assert resp.status_code == 202
+    body = await resp.get_json()
+    assert body == {"queued": True}
+    assert len(called) == 1