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/components/subscriptions/SubscriptionsTab.vue

@@ -1,5 +1,7 @@
 <template>
   <div>
+    <SchedulerStatusBar :status="store.scheduleStatus" class="fc-subs__sched" />
+
     <div class="fc-subs__bar">
       <v-btn color="accent" prepend-icon="mdi-plus" @click="openAddSource(null)">
         Add subscription
@@ -197,7 +199,7 @@
 </template>
 
 <script setup>
-import { computed, onMounted, ref, watch } from 'vue'
+import { computed, onMounted, onUnmounted, ref, watch } from 'vue'
 import { useRoute, useRouter } from 'vue-router'
 import { useSourcesStore } from '../../stores/sources.js'
 import { usePlatformsStore } from '../../stores/platforms.js'
@@ -207,6 +209,7 @@ import SourceHealthDot from './SourceHealthDot.vue'
 import SourceFormDialog from './SourceFormDialog.vue'
 import ArtistCreateDialog from './ArtistCreateDialog.vue'
 import PlatformChip from './PlatformChip.vue'
+import SchedulerStatusBar from './SchedulerStatusBar.vue'
 
 const ITEMS_PER_PAGE_OPTIONS = [
   { value: 25, title: '25' },
@@ -250,14 +253,24 @@ const failureThreshold = computed(() =>
 async function refresh() {
   await store.loadAll()
   await platformsStore.loadAll()
+  store.loadScheduleStatus()
   if (!importStore.settings) await importStore.loadSettings()
 }
 
+// Re-poll scheduler status every 30s so the "last ran" reading + health
+// dot stay honest (the Beat tick fires every 60s).
+let schedId = null
 onMounted(() => {
   refresh()
   if (artistFilter.value != null) {
     expanded.value = [`artist-${artistFilter.value}`]
   }
+  schedId = setInterval(() => {
+    if (!document.hidden) store.loadScheduleStatus()
+  }, 30000)
+})
+onUnmounted(() => {
+  if (schedId) { clearInterval(schedId); schedId = null }
 })
 watch(() => route.query.artist_id, refresh)
 
@@ -500,6 +513,7 @@ async function bulkDelete() {
 </script>
 
 <style scoped>
+.fc-subs__sched { margin-bottom: 10px; }
 .fc-subs__bar {
   display: flex; gap: 0.75rem; align-items: center;
   flex-wrap: wrap;