feat(maintenance): live queue-depth bar under the backfill buttons

Each maintenance button (ML backfill, centroid recompute → ml queue;
thumbnail backfill → thumbnail queue) now shows a status bar with the live
pending count for its Celery queue, so the operator can see work is already
queued/running before re-triggering and piling on. MaintenancePanel polls
/api/system/activity/queues every 4s; QueueStatusBar reads the depth and
turns warning-colored when the queue is busy.

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

frontend/src/components/settings/MLBackfillCard.vue

@@ -10,6 +10,7 @@
         <v-icon start>mdi-refresh</v-icon> Run backfill now
       </v-btn>
       <span v-if="done" class="ml-3 text-caption">Enqueued.</span>
+      <QueueStatusBar queue="ml" queue-label="ML" />
     </v-card-text>
   </v-card>
 </template>
@@ -18,6 +19,7 @@
 import { toast } from '../../utils/toast.js'
 import { ref } from 'vue'
 import { useMLStore } from '../../stores/ml.js'
+import QueueStatusBar from './QueueStatusBar.vue'
 const store = useMLStore()
 const busy = ref(false)
 const done = ref(false)