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>
bvandeusen
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