add: harden thumbnail generation and validate image assets

README.md

@@ -34,6 +34,12 @@ If you only need to regenerate thumbnails, run:
 npm run build:thumbs
 ```
 
+To force a full thumbnail rebuild, run:
+
+```sh
+npm run build:thumbs:force
+```
+
 ## Image Conventions
 
 - Full-size images and thumbnails share the same numeric ID
@@ -45,6 +51,7 @@ npm run build:thumbs
 ## Thumbnail Focus
 
 Thumbnails are generated from `images/fulls` with `sharp` at `240x320`.
+The generator auto-rotates images using EXIF orientation, skips unchanged files by default, and removes stale thumbnail `.jpg` files that no longer map to a meal entry.
 
 For images that should crop away from the center, add optional thumbnail focus metadata to the meal entry: