feat(api): TaxCalculator 도메인 + PaymentService 세금 통합 + AgentService 세무 매핑 (도메인 P0-1-c)

P0-1 세무 처리 마무리. ga-batch 정산 step 통합은 도메인 P0 마무리 단계.

TaxCalculator (신규, ga-api/service/tax):
- @Component pure function: TaxBreakdown calculate(amount, taxType, vatType)
- record TaxBreakdown {incomeTax, localTax, vat, netAmount}
- 세율은 SystemConfigService로 외부화 (TAX_RATE_BUSINESS_INCOME 등)
- 공식:
  - incomeTax = amount × incomeRate / 100 (ROUND_HALF_UP, scale 0)
  - localTax = incomeTax × localRate / 100
  - vat = (TAXABLE) ? amount × vatRate / 100 : 0
  - netAmount = amount - incomeTax - localTax + vat

PaymentService:
- calculateTaxes(paymentId) — 단건 재계산 @Transactional
- calculateTaxesForMonth(settleMonth) — 월별 일괄 @Transactional + batch update

PaymentMapper (ga-core):
- updateTaxes / updateTaxesBatch / selectBySettleMonth 추가

AgentService.toVO():
- taxType/vatType/businessNo 매핑 + default 처리
- 사업소득자인데 businessNo 미입력 시 경고 로그

PaymentController:
- POST /api/payments/{id}/calculate-tax (@RequirePermission, @DataChangeLog)
- POST /api/payments/calculate-taxes?settleMonth=YYYYMM

ga-core/src/main/java/com/ga/core/mapper/settle/PaymentMapper.java

@@ -6,6 +6,7 @@ import com.ga.core.vo.settle.SettleSearchParam;
 import org.apache.ibatis.annotations.Mapper;
 import org.apache.ibatis.annotations.Param;
 
+import java.math.BigDecimal;
 import java.util.List;
 
 @Mapper
@@ -19,7 +20,14 @@ public interface PaymentMapper {
                      @Param("failReason") String failReason);
     int updateTransferFile(@Param("paymentIds") List<Long> paymentIds,
                            @Param("transferFileId") Long transferFileId);
+    int updateTaxes(@Param("paymentId") Long paymentId,
+                    @Param("incomeTaxAmount") BigDecimal incomeTaxAmount,
+                    @Param("localTaxAmount") BigDecimal localTaxAmount,
+                    @Param("vatAmount") BigDecimal vatAmount,
+                    @Param("netAmount") BigDecimal netAmount);
+    int updateTaxesBatch(@Param("list") List<PaymentVO> list);
 
     List<PaymentVO> selectPendingByMonthAndBank(@Param("settleMonth") String settleMonth,
                                                 @Param("bankCode") String bankCode);
+    List<PaymentVO> selectBySettleMonth(@Param("settleMonth") String settleMonth);
 }