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_test.go

@@ -230,6 +230,47 @@ func TestResumableCompleteRejectsMissingChunks(t *testing.T) {
 	}
 }
 
+func TestProcessingResumableFailureMarksBoxFailed(t *testing.T) {
+	service := newTestUploadService(t)
+	session, err := service.CreateResumableSession([]ResumableFileInput{{
+		Name:        "note.txt",
+		Size:        4,
+		ContentType: "text/plain",
+	}}, UploadOptions{MaxDays: 1, StorageBackendID: "missing"}, 4, time.Hour, "")
+	if err != nil {
+		t.Fatalf("CreateResumableSession returned error: %v", err)
+	}
+	if _, err := service.PutResumableChunk(testContext(), session.ID, session.Files[0].ID, 0, strings.NewReader("note")); err != nil {
+		t.Fatalf("PutResumableChunk returned error: %v", err)
+	}
+	result, processing, err := service.CreateProcessingBoxFromResumable(session.ID)
+	if err != nil {
+		t.Fatalf("CreateProcessingBoxFromResumable returned error: %v", err)
+	}
+	if processing.Status != ResumableStatusProcessing {
+		t.Fatalf("session status = %q, want processing", processing.Status)
+	}
+	if _, err := service.FinalizeProcessingResumableSession(testContext(), session.ID); err == nil {
+		t.Fatalf("FinalizeProcessingResumableSession accepted missing backend")
+	}
+	box := getTestBox(t, service, result.BoxID)
+	if len(box.Files) != 1 {
+		t.Fatalf("box files = %+v", box.Files)
+	}
+	if box.Files[0].Processing {
+		t.Fatalf("failed file is still marked processing: %+v", box.Files[0])
+	}
+	if box.Files[0].ProcessingError == "" {
+		t.Fatalf("failed file did not store processing error: %+v", box.Files[0])
+	}
+	if !box.Trouble {
+		t.Fatalf("failed box was not marked as trouble: %+v", box)
+	}
+	if box.TroubleReason == "" {
+		t.Fatalf("failed box did not store trouble reason: %+v", box)
+	}
+}
+
 func TestResumablePartialCompleteKeepsOnlyFinishedFiles(t *testing.T) {
 	service := newTestUploadService(t)
 	session, err := service.CreateResumableSession([]ResumableFileInput{