Separate face detection into standalone daemon

- Strip all face code from update.rb; add shared log helper writing to
  /opt/albumen/log/albumen.log with [update] prefix.  update.rb now owns
  only album.json; face_daemon.rb owns faces.json.
- New scripts/face_daemon.rb: polls MEDIA_ROOT for unprocessed images,
  calls faces.py in batches, writes per-directory faces.json sidecars
  atomically.  Graceful SIGTERM/SIGINT shutdown between directories.
- New config/face_daemon.service: systemd unit running as albumen user,
  Restart=on-failure, logs via SyslogIdentifier=albumen-faces.
- app.rb: add FACES_ENABLED constant; load_faces() helper reads faces.json;
  album_files() merges face data into each entry as :faces field.
- Update README.md and DESIGN.md to document the new daemon architecture,
  faces.json schema, and service management commands.

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

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