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>

frontend/src/stores/downloads.js

@@ -88,10 +88,17 @@ export const useDownloadsStore = defineStore('downloads', () => {
     return activeEvents.value
   }
 
+  // POSTs to the download-recovery sweep endpoint (fire-and-forget — the
+  // Beat schedule also runs it every 5 min). The caller should refresh
+  // failing/stats a few seconds after this resolves to see swept rows.
+  async function recoverStalled() {
+    return api.post('/api/downloads/recover-stalled')
+  }
+
   return {
     events, cursor, hasMore, filter, selected, loading, error, stats,
     activity, failing, activeEvents,
     loadFirst, loadMore, loadOne, applyFilter, closeDetail, loadStats,
-    loadActivity, loadFailing, loadActive,
+    loadActivity, loadFailing, loadActive, recoverStalled,
   }
 })