feat(I3): always-on pipeline status indicator in the top nav

New /api/system/activity/summary aggregates scheduler health + per-queue
pending depths + running count + 24h failure count in one cached call (safe
to poll app-wide). PipelineStatusChip lives in the TopNav on every page: a
compact running/queued/failing chip with a scheduler-health dot that expands
to a popover (scheduler, busy queues, counts, link to downloads). Polls the
summary every 8s, paused when backgrounded. Reuses the queue-cache read via
_queues_cached(). + API test for the summary shape.

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

frontend/src/components/TopNav.vue

@@ -8,6 +8,7 @@
       <span class="fc-health" :title="health.label">
         <v-icon size="x-small" :color="health.color">{{ health.icon }}</v-icon>
       </span>
+      <PipelineStatusChip />
     </div>
 
     <nav class="fc-links">
@@ -29,6 +30,7 @@
 import { computed, onMounted } from 'vue'
 import router, { FRONT_DOOR } from '../router.js'
 import { useSystemStore } from '../stores/system.js'
+import PipelineStatusChip from './PipelineStatusChip.vue'
 
 const system = useSystemStore()
 onMounted(() => system.refreshHealth())