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, 13 insertions, 2 deletions

diff --git a/README.md b/README.md
index a056891..adc19a6 100644
--- a/README.md
+++ b/README.md
@@ -32,6 +32,8 @@ back end, plain HTML/CSS/JS front end.  Live at **https://albumen.jots.org**.
 - Per-album: title, description, cover image (specific file or random), sub-album order, visibility
 - Per-file: caption, visibility
 - Save button at top and bottom of the edit form
+- **Run Update** button scans for new/removed files and generates missing thumbnails;
+  **Force rescan all** checkbox bypasses the sentinel and rescans every directory
 
 ### Media support
 
@@ -84,20 +86,29 @@ The update script walks the media tree, creates/updates `album.json` files with
 EXIF dates and image dimensions, and pre-generates thumbnails.
 
 ```bash
-# On the server — process the entire tree
+# On the server — process the entire tree (skips unchanged directories)
 ruby /opt/albumen/scripts/update.rb
 
-# Process only one album (and its sub-albums)
+# Process only one album (and its sub-albums) — always runs regardless of mtime
 ruby /opt/albumen/scripts/update.rb 2024-Italy
 
 # With an absolute path
 ruby /opt/albumen/scripts/update.rb /var/albumen/2024-Italy
+
+# Force a full rescan of everything, ignoring all change detection
+ruby /opt/albumen/scripts/update.rb --force
 ```
 
 **Resilience guarantees — safe to interrupt and re-run at any point:**
 - `album.json` is written atomically (temp file + rename); no partial writes.
+- Unchanged directories are skipped via a `.albumen_scanned` sentinel file —
+  a global run with nothing new typically completes in under a second.
+- Providing an explicit directory bypasses the sentinel for that subtree, so
+  `update.rb some-album` always rescans that album even if nothing appears changed.
 - Thumbnails that already exist are skipped entirely.
 - EXIF metadata already recorded is not re-extracted.
+- Images with no EXIF data are marked `exif_absent` after the first attempt so
+  exiftool is not re-invoked on them in subsequent rescans.
 - Deleted files are pruned from `album.json` automatically.
 
 Typical workflow: