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/CentroidRecomputeCard.vue

@@ -11,6 +11,7 @@
         <v-icon start>mdi-vector-triangle</v-icon> Recompute centroids
       </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>
@@ -19,6 +20,7 @@
 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)

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)

frontend/src/components/settings/MaintenancePanel.vue

@@ -23,6 +23,8 @@
 </template>
 
 <script setup>
+import { onMounted, onUnmounted } from 'vue'
+
 import MLBackfillCard from './MLBackfillCard.vue'
 import CentroidRecomputeCard from './CentroidRecomputeCard.vue'
 import ThumbnailBackfillCard from './ThumbnailBackfillCard.vue'
@@ -31,6 +33,22 @@ import AllowlistTable from './AllowlistTable.vue'
 import AliasTable from './AliasTable.vue'
 import BackupCard from './BackupCard.vue'
 import LegacyMigrationCard from './LegacyMigrationCard.vue'
+import { useSystemActivityStore } from '../../stores/systemActivity.js'
+
+// Poll queue depths so each card's QueueStatusBar shows live pending
+// counts — the operator can see a backfill is already queued/running
+// before re-triggering it.
+const activity = useSystemActivityStore()
+let qTimer = null
+onMounted(() => {
+  activity.loadQueues()
+  qTimer = setInterval(() => {
+    if (!document.hidden) activity.loadQueues()
+  }, 4000)
+})
+onUnmounted(() => {
+  if (qTimer) { clearInterval(qTimer); qTimer = null }
+})
 </script>
 
 <style scoped>

frontend/src/components/settings/QueueStatusBar.vue

@@ -0,0 +1,69 @@
+<template>
+  <div class="fc-qbar" :class="`fc-qbar--${state}`" :title="hint">
+    <v-icon size="14" class="fc-qbar__icon">{{ icon }}</v-icon>
+    <span>{{ label }}</span>
+  </div>
+</template>
+
+<script setup>
+import { computed } from 'vue'
+
+import { useSystemActivityStore } from '../../stores/systemActivity.js'
+
+const props = defineProps({
+  // Celery queue these buttons feed (see celery_app.task_routes).
+  queue: { type: String, required: true },
+  queueLabel: { type: String, default: '' },
+})
+
+const store = useSystemActivityStore()
+
+// Redis LLEN per queue from /api/system/activity/queues; null when the
+// broker read failed or hasn't loaded yet.
+const depth = computed(() => {
+  const q = store.queues?.queues
+  return q ? (q[props.queue] ?? null) : null
+})
+const state = computed(() => {
+  if (depth.value == null) return 'unknown'
+  return depth.value > 0 ? 'busy' : 'idle'
+})
+const icon = computed(() => ({
+  busy: 'mdi-progress-clock',
+  idle: 'mdi-check-circle-outline',
+  unknown: 'mdi-help-circle-outline',
+}[state.value]))
+const qname = computed(() => props.queueLabel || props.queue)
+const label = computed(() => {
+  if (depth.value == null) return `${qname.value} queue · status unavailable`
+  if (depth.value === 0) return `${qname.value} queue idle — nothing pending`
+  return `${depth.value} pending in the ${qname.value} queue`
+})
+const hint = computed(() =>
+  state.value === 'busy'
+    ? `The ${qname.value} queue already has ${depth.value} task(s) waiting — running again now just adds to that backlog.`
+    : '',
+)
+</script>
+
+<style scoped>
+.fc-qbar {
+  display: flex; align-items: center; gap: 6px;
+  margin-top: 12px;
+  padding: 5px 10px;
+  border-radius: 6px;
+  font-size: 0.78rem;
+  font-variant-numeric: tabular-nums;
+}
+.fc-qbar__icon { flex: 0 0 auto; }
+.fc-qbar--idle,
+.fc-qbar--unknown {
+  color: rgb(var(--v-theme-on-surface-variant));
+  background: rgb(var(--v-theme-on-surface) / 0.04);
+}
+.fc-qbar--busy {
+  color: rgb(var(--v-theme-warning));
+  background: rgb(var(--v-theme-warning) / 0.12);
+  font-weight: 600;
+}
+</style>

frontend/src/components/settings/ThumbnailBackfillCard.vue

@@ -11,6 +11,7 @@
         <v-icon start>mdi-image-refresh</v-icon> Run backfill now
       </v-btn>
       <span v-if="done" class="ml-3 text-caption">Enqueued.</span>
+      <QueueStatusBar queue="thumbnail" queue-label="Thumbnail" />
     </v-card-text>
   </v-card>
 </template>
@@ -19,6 +20,7 @@
 import { toast } from '../../utils/toast.js'
 import { ref } from 'vue'
 import { useThumbnailsStore } from '../../stores/thumbnails.js'
+import QueueStatusBar from './QueueStatusBar.vue'
 const store = useThumbnailsStore()
 const busy = ref(false)
 const done = ref(false)