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/libs/jobs/thumbnails_test.go

@@ -50,6 +50,36 @@ func TestGenerateMissingThumbnailsForBox(t *testing.T) {
 	}
 }
 
+func TestGenerateMissingThumbnailsForTroubleBoxSkipsWork(t *testing.T) {
+	service := newThumbnailTestUploadService(t)
+	result := createThumbnailTestBox(t, service)
+	box, err := service.GetBox(result.BoxID)
+	if err != nil {
+		t.Fatalf("GetBox returned error: %v", err)
+	}
+	box.Trouble = true
+	box.TroubleReason = "storage backend failed"
+	if err := service.SaveBox(box); err != nil {
+		t.Fatalf("SaveBox returned error: %v", err)
+	}
+
+	jobResult, err := generateMissingThumbnailsForBox(service, slog.New(slog.NewTextHandler(io.Discard, nil)), box)
+	if err != nil {
+		t.Fatalf("generateMissingThumbnailsForBox returned error: %v", err)
+	}
+	if jobResult != (ThumbnailJobResult{}) {
+		t.Fatalf("job result = %+v, want no work for trouble box", jobResult)
+	}
+
+	updated, err := service.GetBox(result.BoxID)
+	if err != nil {
+		t.Fatalf("GetBox after job returned error: %v", err)
+	}
+	if updated.Files[0].Thumbnail != "" {
+		t.Fatalf("thumbnail was generated for trouble box: %+v", updated.Files[0])
+	}
+}
+
 func TestCreateTextThumbnailRendersMarkdownAsJPEG(t *testing.T) {
 	data, err := createTextThumbnail(services.File{
 		Name:        "notes.md",