feat(backend): handle processing errors and add PWA routes

- Block file downloads and previews with a 424 StatusFailedDependency if file processing failed or the box has issues.
- Register routes for `/service-worker.js` and `/share-target` to support PWA features.
- Update README.md with an AI usage disclosure.

README.md

@@ -357,3 +357,9 @@ bbolt database and JSON logs always remain local under `./data/db` and `./data/l
 
 The static handler sets long-lived immutable caching for images, video, audio, and fonts, shorter
 caching for CSS/JS, and gzip compression for compressible responses.
+
+## AI Usage
+
+I have used AI to accelerate development, all of the code has been reviewed by humans. I have mostly used self-hosted models as well as big models from big companies for a monthly subscription fee. 
+
+I have nothing against AI as long as you can tell me what every single line of your code does. That's how I personally view things.