#306

docs/commands.md

@@ -10,6 +10,15 @@ bundle exec rspec
 bundle exec rails routes
 ```
 
+Material video thumbnail generation requires `ffmpeg` on the host. If `ffmpeg`
+is missing or frame extraction fails, `MaterialThumbnailGenerator` logs the
+result and stderr, then leaves the existing thumbnail unchanged.
+
+```sh
+cd backend
+bundle exec rails materials:thumbnails:backfill
+```
+
 ## Frontend
 
 ```sh