Speed up update.rb and fix UI always forcing full rescan

- update.rb: skip exiftool on images marked exif_absent (set after first
  failed attempt); prevents repeated slow scans of old photos with no EXIF
- update.rb: explicit directory argument now implies force — passing a path
  always rescans that subtree regardless of sentinel mtime
- app.rb: /admin/update no longer hardcodes --force; sentinel-based skipping
  is used by default, making UI updates finish in seconds instead of minutes
- admin/album.erb: add "Force rescan all" checkbox to Run Update button;
  checked state passes force=1 to the server and restores --force behavior
- README.md, DESIGN.md: document sentinel skipping, exif_absent flag, and
  explicit-directory force behavior

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

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