Scan for visual duplicates

Walks every photo under s3://ai-content-assets/planeta-alofoke/assets/people/, computes an 8×8 average-hash (aHash), and clusters images whose Hamming distance is ≤ the threshold. Cluster report writes to dashboard-data/asset-duplicates.json. With --confirm, also patches brain/asset-constraints.json so the slide-plan auto-skips them on the next build.