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/FailingSourcesCard.vue

@@ -0,0 +1,87 @@
+<template>
+  <v-card v-if="sources.length" variant="tonal" color="error" class="fc-fail mb-4">
+    <div class="fc-fail__head" @click="open = !open">
+      <v-icon size="small">mdi-alert-circle</v-icon>
+      <span class="fc-fail__title">
+        {{ sources.length }}
+        {{ sources.length === 1 ? 'source is' : 'sources are' }} failing
+      </span>
+      <v-spacer />
+      <v-btn
+        size="small" variant="text" prepend-icon="mdi-refresh"
+        :loading="retryingAll"
+        @click.stop="$emit('retry-all', sources)"
+      >
+        Retry all
+      </v-btn>
+      <v-icon size="small">{{ open ? 'mdi-chevron-up' : 'mdi-chevron-down' }}</v-icon>
+    </div>
+
+    <v-expand-transition>
+      <div v-if="open" class="fc-fail__body">
+        <div v-for="s in sources" :key="s.id" class="fc-fail__row">
+          <PlatformChip :platform="s.platform" size="x-small" />
+          <span class="fc-fail__artist">{{ s.artist_name || `#${s.artist_id}` }}</span>
+          <v-chip size="x-small" color="error" variant="flat" label class="fc-fail__count">
+            {{ s.consecutive_failures }}× failed
+          </v-chip>
+          <span class="fc-fail__err" :title="s.last_error || ''">
+            {{ s.last_error || 'no error message recorded' }}
+          </span>
+          <v-spacer />
+          <v-btn
+            size="x-small" variant="text" prepend-icon="mdi-refresh"
+            :loading="retryingIds.has(s.id)"
+            @click="$emit('retry', s)"
+          >
+            Retry
+          </v-btn>
+        </div>
+      </div>
+    </v-expand-transition>
+  </v-card>
+</template>
+
+<script setup>
+import { ref } from 'vue'
+import PlatformChip from './PlatformChip.vue'
+
+defineProps({
+  // source records from /api/sources?failing=true
+  sources: { type: Array, default: () => [] },
+  retryingIds: { type: Set, default: () => new Set() },
+  retryingAll: { type: Boolean, default: false },
+})
+defineEmits(['retry', 'retry-all'])
+
+const open = ref(true)
+</script>
+
+<style scoped>
+.fc-fail { border-radius: 8px; }
+.fc-fail__head {
+  display: flex; align-items: center; gap: 8px;
+  padding: 10px 14px;
+  cursor: pointer;
+  user-select: none;
+}
+.fc-fail__title { font-weight: 600; }
+.fc-fail__body {
+  padding: 0 14px 10px;
+  display: flex; flex-direction: column; gap: 2px;
+}
+.fc-fail__row {
+  display: flex; align-items: center; gap: 10px;
+  padding: 6px 8px;
+  border-radius: 4px;
+  background: rgb(var(--v-theme-surface) / 0.4);
+}
+.fc-fail__artist { font-weight: 600; white-space: nowrap; }
+.fc-fail__count { flex: 0 0 auto; }
+.fc-fail__err {
+  color: rgb(var(--v-theme-on-surface-variant));
+  font-size: 0.8rem;
+  overflow: hidden; text-overflow: ellipsis; white-space: nowrap;
+  max-width: 46ch;
+}
+</style>