feat(ia): wave 2 — Activity becomes the whole-app pulse; Overview gets the health strip

The Activity tab only knew Celery — the GPU agent (the majority of processing)
and the download pipeline were invisible there. Two new self-polling panels:

- GpuActivityPanel: queue depths + triage verdicts (defects / file-ok /
  unprobed, top reason buckets) with a jump to Maintenance -> Failed
  processing. The triage detail refetches only when the error count moves.
- DownloadsActivityPanel: 24h stat chips + failing-source names with a jump
  into Subscriptions.

Both panels join the Activity tab under Queues+workers AND double as the
Overview health strip (side-by-side grid under the Celery summary) — one
component set, so Overview answers 'is everything healthy?' across all
systems. SystemStatsCards reviewed: content still accurate, left as-is.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

frontend/src/components/settings/SystemActivityTab.vue

@@ -17,6 +17,12 @@
       </v-card-text>
     </v-card>
 
+    <!-- The non-Celery halves of the app (2026-07-02): the GPU agent does the
+         majority of processing and downloads feed the library — Activity is
+         the whole-app pulse, not just the worker queues. -->
+    <GpuActivityPanel @open-maintenance="$emit('open-maintenance')" />
+    <DownloadsActivityPanel />
+
     <!-- Recent failures pane -->
     <v-card class="mb-4">
       <CardHeading icon="mdi-alert-circle-outline" title="Recent failures (last 24h)">
@@ -167,6 +173,10 @@ import { formatRelative as fmtRelative } from '../../utils/date.js'
 import ErrorDetailModal from '../common/ErrorDetailModal.vue'
 import QueuesTable from './QueuesTable.vue'
 import CardHeading from '../common/CardHeading.vue'
+import GpuActivityPanel from './GpuActivityPanel.vue'
+import DownloadsActivityPanel from './DownloadsActivityPanel.vue'
+
+defineEmits(['open-maintenance'])
 
 // Click-to-open modal for full error text. Replaces the unusable
 // :title="..." tooltip (operator-flagged 2026-05-26: SQLAlchemy