feat: add payment card grouping and analysis

lib/models/payment_card_suggestion.dart

@@ -0,0 +1,14 @@
+/// SMS 스캔 등에서 추출한 결제수단 정보 제안
+class PaymentCardSuggestion {
+  final String issuerName;
+  final String? last4;
+  final String? source; // 예: SMS, OCR 등
+
+  const PaymentCardSuggestion({
+    required this.issuerName,
+    this.last4,
+    this.source,
+  });
+
+  bool get hasLast4 => last4 != null && last4!.length == 4;
+}