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/services/upload.go

@@ -117,6 +117,8 @@ type Box struct {
 	Obfuscate        bool      `json:"obfuscate"`
 	CreatorIP        string    `json:"creatorIp,omitempty"`
 	StorageBackendID string    `json:"storageBackendId,omitempty"`
+	Trouble          bool      `json:"trouble,omitempty"`
+	TroubleReason    string    `json:"troubleReason,omitempty"`
 	Files            []File    `json:"files"`
 }
 
@@ -139,6 +141,37 @@ type File struct {
 	UploadedAt              time.Time `json:"uploadedAt"`
 }
 
+func BoxHasTrouble(box Box) bool {
+	if box.Trouble || strings.TrimSpace(box.TroubleReason) != "" {
+		return true
+	}
+	for _, file := range box.Files {
+		if FileHasTrouble(file) {
+			return true
+		}
+	}
+	return false
+}
+
+func BoxTroubleReason(box Box) string {
+	if strings.TrimSpace(box.TroubleReason) != "" {
+		return box.TroubleReason
+	}
+	for _, file := range box.Files {
+		if strings.TrimSpace(file.ProcessingError) != "" {
+			return file.ProcessingError
+		}
+	}
+	if box.Trouble {
+		return "box has failed processing"
+	}
+	return ""
+}
+
+func FileHasTrouble(file File) bool {
+	return strings.TrimSpace(file.ProcessingError) != ""
+}
+
 type UploadResult struct {
 	BoxID        string       `json:"boxId"`
 	BoxURL       string       `json:"boxUrl"`