feat: add ScanRecord model with toJson

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/models/scan_record_test.dart

@@ -0,0 +1,29 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pad_scanner/models/scan_record.dart';
+
+void main() {
+  group('ScanRecord', () {
+    test('creates with correct defaults', () {
+      final record = ScanRecord(
+        barcode: '1234567890',
+        codeType: 'CODE128',
+        timestamp: DateTime.parse('2026-05-07T10:30:00Z'),
+      );
+      expect(record.barcode, '1234567890');
+      expect(record.codeType, 'CODE128');
+      expect(record.status, SendStatus.pending);
+    });
+
+    test('toJson produces correct map', () {
+      final record = ScanRecord(
+        barcode: 'ABC123',
+        codeType: 'QR',
+        timestamp: DateTime.parse('2026-05-07T10:30:00Z'),
+      );
+      final json = record.toJson();
+      expect(json['barcode'], 'ABC123');
+      expect(json['code_type'], 'QR');
+      expect(json['timestamp'], '2026-05-07T10:30:00.000Z');
+    });
+  });
+}