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-api/src/main/java/com/ga/api/controller/settle/PaymentController.java

@@ -48,4 +48,23 @@ public class PaymentController {
             @RequestParam String bankCode) {
         return ApiResponse.ok(paymentService.generateTransferFile(settleMonth, bankCode));
     }
+
+    @Operation(summary = "세금 단건 재계산")
+    @PostMapping("/{paymentId}/calculate-tax")
+    @RequirePermission(menu = "PAYMENT", perm = "UPDATE")
+    @DataChangeLog(menu = "PAYMENT", table = "payment")
+    public ApiResponse<Void> calculateTax(@PathVariable Long paymentId) {
+        paymentService.calculateTaxes(paymentId);
+        return ApiResponse.ok();
+    }
+
+    @Operation(summary = "세금 월별 일괄 재계산")
+    @PostMapping("/calculate-taxes")
+    @RequirePermission(menu = "PAYMENT", perm = "UPDATE")
+    @DataChangeLog(menu = "PAYMENT", table = "payment")
+    public ApiResponse<Void> calculateTaxesForMonth(
+            @Pattern(regexp = "\\d{6}", message = "settleMonth는 YYYYMM 형식이어야 합니다") @RequestParam String settleMonth) {
+        paymentService.calculateTaxesForMonth(settleMonth);
+        return ApiResponse.ok();
+    }
 }

ga-api/src/main/java/com/ga/api/service/org/AgentService.java

@@ -7,11 +7,15 @@ import com.ga.common.model.PageResponse;
 import com.ga.core.mapper.org.AgentMapper;
 import com.ga.core.vo.org.*;
 import com.ga.core.vo.org.AgentStatus;
+import com.ga.core.vo.org.TaxType;
+import com.ga.core.vo.org.VatType;
 import lombok.RequiredArgsConstructor;
+import lombok.extern.slf4j.Slf4j;
 import org.apache.ibatis.cursor.Cursor;
 import org.springframework.stereotype.Service;
 import org.springframework.transaction.annotation.Transactional;
 
+@Slf4j
 @Service
 @RequiredArgsConstructor
 public class AgentService {
@@ -102,6 +106,17 @@ public class AgentService {
         vo.setBankCode(req.getBankCode());
         vo.setAccountNo(req.getAccountNo());
         vo.setJoinDate(req.getJoinDate());
+
+        String taxType = req.getTaxType() != null ? req.getTaxType() : TaxType.BUSINESS_INCOME.name();
+        String vatType = req.getVatType() != null ? req.getVatType() : VatType.NONE.name();
+        vo.setTaxType(taxType);
+        vo.setVatType(vatType);
+        vo.setBusinessNo(req.getBusinessNo());
+
+        if (TaxType.BUSINESS_INCOME.name().equals(taxType)
+                && (req.getBusinessNo() == null || req.getBusinessNo().isBlank())) {
+            log.warn("agentId={} taxType=BUSINESS_INCOME이지만 businessNo가 비어있습니다", agentId);
+        }
         return vo;
     }
 }

ga-api/src/main/java/com/ga/api/service/settle/PaymentService.java

@@ -1,21 +1,36 @@
 package com.ga.api.service.settle;
 
+import com.ga.api.service.tax.TaxBreakdown;
+import com.ga.api.service.tax.TaxCalculator;
 import com.ga.api.service.transfer.TransferFileService;
+import com.ga.common.exception.BizException;
+import com.ga.common.exception.ErrorCode;
 import com.ga.common.model.PageResponse;
+import com.ga.core.mapper.org.AgentMapper;
 import com.ga.core.mapper.settle.PaymentMapper;
+import com.ga.core.vo.org.AgentVO;
+import com.ga.core.vo.org.TaxType;
+import com.ga.core.vo.org.VatType;
 import com.ga.core.vo.settle.PaymentResp;
+import com.ga.core.vo.settle.PaymentVO;
 import com.ga.core.vo.settle.SettleSearchParam;
 import com.ga.core.vo.settle.TransferFileResp;
 import lombok.RequiredArgsConstructor;
+import lombok.extern.slf4j.Slf4j;
 import org.springframework.stereotype.Service;
 import org.springframework.transaction.annotation.Transactional;
 
+import java.util.List;
+
+@Slf4j
 @Service
 @RequiredArgsConstructor
 public class PaymentService {
 
     private final PaymentMapper mapper;
+    private final AgentMapper agentMapper;
     private final TransferFileService transferFileService;
+    private final TaxCalculator taxCalculator;
 
     @Transactional(readOnly = true)
     public PageResponse<PaymentResp> list(SettleSearchParam param) {
@@ -31,4 +46,55 @@ public class PaymentService {
     public TransferFileResp generateTransferFile(String settleMonth, String bankCode) {
         return transferFileService.generate(settleMonth, bankCode);
     }
+
+    @Transactional
+    public void calculateTaxes(Long paymentId) {
+        PaymentVO payment = mapper.selectById(paymentId);
+        if (payment == null) throw new BizException(ErrorCode.NOT_FOUND);
+        applyTaxes(payment);
+        mapper.updateTaxes(payment.getPaymentId(),
+                payment.getIncomeTaxAmount(), payment.getLocalTaxAmount(),
+                payment.getVatAmount(), payment.getNetAmount());
+    }
+
+    @Transactional
+    public void calculateTaxesForMonth(String settleMonth) {
+        List<PaymentVO> payments = mapper.selectBySettleMonth(settleMonth);
+        for (PaymentVO payment : payments) {
+            applyTaxes(payment);
+        }
+        if (!payments.isEmpty()) {
+            mapper.updateTaxesBatch(payments);
+        }
+    }
+
+    private void applyTaxes(PaymentVO payment) {
+        AgentVO agent = agentMapper.selectById(payment.getAgentId());
+        TaxType taxType = parseTaxType(agent);
+        VatType vatType = parseVatType(agent);
+
+        if (taxType == TaxType.BUSINESS_INCOME && (agent == null || agent.getBusinessNo() == null || agent.getBusinessNo().isBlank())) {
+            log.warn("agentId={} taxType=BUSINESS_INCOME이지만 businessNo가 비어있습니다", payment.getAgentId());
+        }
+
+        TaxBreakdown breakdown = taxCalculator.calculate(payment.getPayAmount(), taxType, vatType);
+        payment.setIncomeTaxAmount(breakdown.incomeTax());
+        payment.setLocalTaxAmount(breakdown.localTax());
+        payment.setVatAmount(breakdown.vat());
+        payment.setNetAmount(breakdown.netAmount());
+    }
+
+    private TaxType parseTaxType(AgentVO agent) {
+        if (agent != null && agent.getTaxType() != null) {
+            try { return TaxType.valueOf(agent.getTaxType()); } catch (IllegalArgumentException ignored) {}
+        }
+        return TaxType.BUSINESS_INCOME;
+    }
+
+    private VatType parseVatType(AgentVO agent) {
+        if (agent != null && agent.getVatType() != null) {
+            try { return VatType.valueOf(agent.getVatType()); } catch (IllegalArgumentException ignored) {}
+        }
+        return VatType.NONE;
+    }
 }

ga-api/src/main/java/com/ga/api/service/tax/TaxBreakdown.java

@@ -0,0 +1,10 @@
+package com.ga.api.service.tax;
+
+import java.math.BigDecimal;
+
+public record TaxBreakdown(
+        BigDecimal incomeTax,
+        BigDecimal localTax,
+        BigDecimal vat,
+        BigDecimal netAmount
+) {}

ga-api/src/main/java/com/ga/api/service/tax/TaxCalculator.java

@@ -0,0 +1,41 @@
+package com.ga.api.service.tax;
+
+import com.ga.common.system.SystemConfigService;
+import com.ga.core.vo.org.TaxType;
+import com.ga.core.vo.org.VatType;
+import lombok.RequiredArgsConstructor;
+import org.springframework.stereotype.Component;
+
+import java.math.BigDecimal;
+import java.math.RoundingMode;
+
+@Component
+@RequiredArgsConstructor
+public class TaxCalculator {
+
+    private final SystemConfigService configService;
+
+    public TaxBreakdown calculate(BigDecimal amount, TaxType taxType, VatType vatType) {
+        BigDecimal incomeRate = resolveIncomeRate(taxType);
+        BigDecimal localRate  = configService.getDecimal("TAX_RATE_LOCAL_INCOME", new BigDecimal("10.0"));
+        BigDecimal vatRate    = configService.getDecimal("TAX_RATE_VAT", new BigDecimal("10.0"));
+
+        BigDecimal incomeTax = amount.multiply(incomeRate)
+                .divide(BigDecimal.valueOf(100), 0, RoundingMode.HALF_UP);
+        BigDecimal localTax = incomeTax.multiply(localRate)
+                .divide(BigDecimal.valueOf(100), 0, RoundingMode.HALF_UP);
+        BigDecimal vat = VatType.TAXABLE.equals(vatType)
+                ? amount.multiply(vatRate).divide(BigDecimal.valueOf(100), 0, RoundingMode.HALF_UP)
+                : BigDecimal.ZERO;
+        BigDecimal netAmount = amount.subtract(incomeTax).subtract(localTax).add(vat);
+
+        return new TaxBreakdown(incomeTax, localTax, vat, netAmount);
+    }
+
+    private BigDecimal resolveIncomeRate(TaxType taxType) {
+        if (TaxType.OTHER_INCOME.equals(taxType)) {
+            return configService.getDecimal("TAX_RATE_OTHER_INCOME", new BigDecimal("8.8"));
+        }
+        return configService.getDecimal("TAX_RATE_BUSINESS_INCOME", new BigDecimal("3.3"));
+    }
+}