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.

backend/static/site.webmanifest

@@ -7,6 +7,22 @@
   "display": "standalone",
   "background_color": "#0b0b16",
   "theme_color": "#8b5cf6",
+  "share_target": {
+    "action": "/share-target",
+    "method": "POST",
+    "enctype": "multipart/form-data",
+    "params": {
+      "title": "title",
+      "text": "text",
+      "url": "url",
+      "files": [
+        {
+          "name": "files",
+          "accept": ["*/*"]
+        }
+      ]
+    }
+  },
   "icons": [
     {
       "src": "/static/android-chrome-192x192.png",