feat(dashboards): scheduler health strip, failing-source rollup, 24h activity sparkline, credential staleness nudge

D1 scheduler visibility: AppSetting last-tick stamp on every Beat tick +
GET /api/sources/schedule-status (last_tick_at/next_due_at/due_now/auto_sources)
+ SchedulerStatusBar on the Subscriptions tab (re-polled every 30s).

D2 failing-source rollup: ?failing=true on the sources list + FailingSourcesCard
on Downloads with per-source and bulk "retry" (re-runs the feed via /check).

D3 activity sparkline: GET /api/downloads/activity hourly buckets + CSS bar
chart by the stat chips (failures stacked in error color); refreshes on live poll.

D4 credential staleness: surface last_verified age + "re-verify recommended"
warning past 30d; also fixes the dead last_verified_at field-name mismatch so
the verification row renders at all.

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

frontend/src/stores/downloads.js

@@ -16,6 +16,8 @@ export const useDownloadsStore = defineStore('downloads', () => {
   const loading = ref(false)
   const error = ref(null)
   const stats = ref({ pending: 0, running: 0, ok: 0, error: 0, skipped: 0 })
+  const activity = ref({ hours: 24, buckets: [] })
+  const failing = ref([])
 
   function _params(extra = {}) {
     const out = { limit: 50, ...extra }
@@ -74,8 +76,20 @@ export const useDownloadsStore = defineStore('downloads', () => {
     return stats.value
   }
 
+  async function loadActivity(hours = 24) {
+    activity.value = await api.get('/api/downloads/activity', { params: { hours } })
+    return activity.value
+  }
+
+  async function loadFailing() {
+    failing.value = await api.get('/api/sources', { params: { failing: true } })
+    return failing.value
+  }
+
   return {
     events, cursor, hasMore, filter, selected, loading, error, stats,
+    activity, failing,
     loadFirst, loadMore, loadOne, applyFilter, closeDetail, loadStats,
+    loadActivity, loadFailing,
   }
 })