[Fix]: 카드 등록 오류 수정

- cardBrand -> brand 변수명 변경
- 필요없는 주석 삭제
- 카드 끝 4자리만 나오게 변경

오류 코드 제대로 봐야겠습니다.....

Author: dlawhd

src/main/java/com/backend/domain/payment/dto/request/PaymentMethodCreateRequest.java

@@ -29,6 +29,5 @@ public class PaymentMethodCreateRequest {
     private String bankName;    // 예: KB국민은행
     private String acctLast4;   // "5678"
 
-    @NotBlank
-    private String provider;    // "toss", "iamport" 등..
+    private String provider;    // "toss"
 }

src/main/java/com/backend/domain/payment/dto/response/TossIssueBillingKeyResponse.java

@@ -13,7 +13,7 @@ public class TossIssueBillingKeyResponse {
     private String provider;   // "toss"
 
     @Schema(description = "카드 브랜드/발급사", example = "SHINHAN")
-    private String cardBrand;  // 선택: 스냅샷에 쓰려면
+    private String brand;  // 선택: 스냅샷에 쓰려면
 
     @Schema(description = "카드 끝 4자리", example = "1234")
     private String last4; // 선택

src/main/java/com/backend/domain/payment/entity/PaymentMethod.java

@@ -59,7 +59,7 @@ public class PaymentMethod extends BaseEntity {
     private String acctLast4;                                          // 계좌번호 끝 4자리(예: 5678)..
 
     @Column(length = 32, nullable = false)
-    private String provider;                                          // 어떤 PG를 통해 결제하는지(예: "toss", "iamport")..
+    private String provider;                                          // 어떤 PG를 통해 결제하는지(예: "toss")..
 
     @Column(nullable = false)
     @Builder.Default

src/main/java/com/backend/domain/payment/service/PaymentMethodService.java

@@ -77,6 +77,13 @@ public PaymentMethodResponse create(Long memberId, PaymentMethodCreateRequest re
                 if (isBlank(req.getBrand()) || isBlank(req.getLast4())){
                     throw new IllegalArgumentException("CARD는 brand, last4가 필요합니다. (expMonth/expYear는 선택)");
                 }
+
+                String normalizedLast4 = req.getLast4().replaceAll("\\D", "");
+                if (normalizedLast4.length() < 4) {
+                    throw new IllegalArgumentException("last4는 카드번호 끝 4자리여야 합니다.");
+                }
+                last4 = normalizedLast4.substring(normalizedLast4.length() - 4);
+
                 // 카드 필드 채우기..
                 brand = req.getBrand();
                 last4 = req.getLast4();
@@ -105,6 +112,13 @@ public PaymentMethodResponse create(Long memberId, PaymentMethodCreateRequest re
             }
         }
 
+        String provider = req.getProvider();
+        if (provider == null || provider.isBlank()) {
+            provider = "toss";
+        } else {
+            provider = provider.trim().toLowerCase();
+        }
+
         PaymentMethod entity = PaymentMethod.builder()
                 .member(member)
                 .methodType(type)
@@ -120,7 +134,7 @@ public PaymentMethodResponse create(Long memberId, PaymentMethodCreateRequest re
                 .bankCode(bankCode)
                 .bankName(bankName)
                 .acctLast4(acctLast4)
-                .provider( nvlBlankToNull(req.getProvider()) )
+                .provider(provider)
                 .active(true)
                 .deleted(false)
                 .build();

src/main/java/com/backend/domain/payment/service/TossBillingClientService.java

@@ -89,7 +89,7 @@ public TossIssueBillingKeyResponse issueBillingKey(String customerKey, String au
 
             return TossIssueBillingKeyResponse.builder()
                     .billingKey(billingKey)
-                    .cardBrand(cardBrand)
+                    .brand(cardBrand)
                     .last4(cardNumber)
                     .expMonth(expMonth)
                     .expYear(expYear)

src/main/resources/static/payments/toss/billing-success.html

@@ -36,8 +36,11 @@ <h2>빌링키 발급 완료</h2>
         log(`✓ 교환 성공 → billingKey=${billingKey}`);
 
         // 카드 정보(있으면) 보조로 사용
-        const brand   = d.cardBrand ?? null;
-        const last4   = d.cardNumber ? String(d.cardNumber).slice(-4) : null;
+        const brand   = d.brand ?? d.cardBrand ?? null;
+        const rawLast4 = d.last4 ?? d.cardNumber ?? null;
+        const last4   = rawLast4
+            ? String(rawLast4).replace(/\D/g, '').slice(-4)  // 숫자만 남기고 끝 4자리
+            : null;
         const expMon  = d.expMonth ?? null;
         const expYear = d.expYear  ?? null;