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>

1 files changed, 31 insertions, 5 deletions

diff --git a/scripts/update.rb b/scripts/update.rb
index 9953505..822405f 100644
--- a/scripts/update.rb
+++ b/scripts/update.rb
@@ -1,9 +1,12 @@
 #!/usr/bin/env ruby
 # frozen_string_literal: true
 #
-# Usage: ruby update.rb [relative/path]
-#   Without argument: process entire MEDIA_ROOT tree.
-#   With argument:    process only that subdirectory (and its children).
+# Usage: ruby update.rb [--force] [relative/path]
+#   Without argument: process entire MEDIA_ROOT tree, skipping directories
+#                     whose mtime hasn't changed since the last scan.
+#   With argument:    process only that subdirectory (and its children),
+#                     always scanning regardless of mtime (explicit request).
+#   --force:          scan entire tree ignoring all mtime sentinels.
 #
 # Resilience guarantees:
 #   - album.json is written atomically (temp-file + rename), so a crash
@@ -11,6 +14,8 @@
 #   - Thumbnails are checked before generation; already-done work is skipped.
 #   - EXIF and dimension extraction are skipped if already recorded.
 #   - Safe to re-run at any time; all operations are idempotent.
+#   - Unchanged directories are skipped via a .albumen_scanned sentinel file;
+#     pass --force to bypass.
 
 require 'json'
 require 'yaml'
@@ -27,12 +32,25 @@ VIDEO_EXTS     = %w[mp4 mov avi mkv webm m4v ogv].freeze
 AUDIO_EXTS     = %w[mp3 flac ogg wav m4a aac].freeze
 MEDIA_EXTS     = (IMAGE_EXTS + VIDEO_EXTS + AUDIO_EXTS).freeze
 TRANSCODE_EXTS = %w[avi mkv mov].freeze  # not universally browser-playable; convert to MP4
+SENTINEL_FILE  = '.albumen_scanned'.freeze
+
+# Explicit directory argument implies force — you asked for it, it should run.
+FORCE_UPDATE = !!(ARGV.delete('--force') || ARGV[0])
 
 # ── Directory processing ───────────────────────────────────────────────────────
 
 def process_dir(dir)
   rel = dir.delete_prefix(MEDIA_ROOT).delete_prefix('/')
   label = rel.empty? ? '(root)' : rel
+
+  unless FORCE_UPDATE
+    sentinel = File.join(dir, SENTINEL_FILE)
+    if File.exist?(sentinel) && File.mtime(sentinel) >= File.mtime(dir)
+      puts "Skipping #{label} (unchanged)"
+      return
+    end
+  end
+
   puts "Scanning #{label}"
 
   json_path = File.join(dir, 'album.json')
@@ -102,13 +120,15 @@ def process_dir(dir)
   end
 
   atomic_write_json(json_path, data)
+  FileUtils.touch(File.join(dir, SENTINEL_FILE))
 end
 
 # ── Metadata enrichment ────────────────────────────────────────────────────────
 
 def enrich_image(full, name, meta)
-  needs_exif = meta['taken_at'].nil? || meta['camera'].nil? ||
-               meta['aperture'].nil? || meta['shutter'].nil? || meta['iso'].nil?
+  needs_exif = !meta['exif_absent'] &&
+               (meta['taken_at'].nil? || meta['camera'].nil? ||
+                meta['aperture'].nil? || meta['shutter'].nil? || meta['iso'].nil?)
   if needs_exif
     begin
       exif = MiniExiftool.new(full, numerical: false)
@@ -133,6 +153,12 @@ def enrich_image(full, name, meta)
     rescue StandardError => e
       warn "  #{name}: EXIF error — #{e.message}"
     end
+
+    # If exiftool found nothing at all, record that so we don't retry on every re-scan.
+    if meta['taken_at'].nil? && meta['camera'].nil? &&
+       meta['aperture'].nil? && meta['shutter'].nil? && meta['iso'].nil?
+      meta['exif_absent'] = true
+    end
   end
 
   # Dimensions (skip if already recorded)