On /people/:slug, admins see a sticky left panel and selectable photo
tiles. Clicking a tile (or its checkbox overlay) toggles selection;
clicking without modifier still opens the photo in a new tab. The panel
shows the selection count and two actions:

- Reassign to person: moves the selected photos' face entries from the
  current person's cluster to the chosen person.
- Move to album: moves the photo files on disk and updates album.json,
  faces.json, and people.json rel paths accordingly. Album paths are
  offered via a datalist autocomplete.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

- scripts/cluster_faces.py: greedy centroid clustering (numpy) with 3
  refinement passes; preserves existing UUID/name mappings across re-runs;
  writes MEDIA_ROOT/people.json atomically.
- app.rb: GET /face/* serves cropped+padded face thumbnails (100x100,
  cached under cache/faces/); GET|POST /admin/people for cluster
  management; POST /admin/people/recluster runs cluster_faces.py as a
  background job; POST /admin/people/:uuid saves names+slugs; GET /people
  public grid of named people; GET /people/:slug photos for one person.
- views/admin/people.erb: lists all clusters (named first, then by size),
  face crop samples, inline name form, re-cluster button with live log.
- views/people.erb: public grid of named people.
- views/person.erb: photo grid for one person, linking back to album
  lightbox for each photo.
- views/layout.erb: People link in nav (conditional on FACES_ENABLED).
- public/css/style.css: styles for people admin list and public tiles.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>