feat: 子ども・子育て支援金を給与・賞与計算に追加（令和8年4月以降）

backend/app/payroll/calculation/service.py

@@ -334,9 +334,11 @@ class PayrollCalculationService:
                 
                 if result:
                     standard_monthly_salary = Decimal(str(result["monthly_amount"]))
-                    print(f"[DEBUG] 標準報酬月額表検索: 総支給額={total_payment} → 標準報酬月額={standard_monthly_salary}")
+                    child_support_full = Decimal(str(result["child_support"])) if result["child_support"] is not None else Decimal("0")
+                    print(f"[DEBUG] 標準報酬月額表検索: 総支給額={total_payment} → 標準報酬月額={standard_monthly_salary}, 子育て支援金全額={child_support_full}")
                 else:
                     print(f"[DEBUG] 標準報酬月額表検索: 該当等級なし（総支給額={total_payment}）")
+                    child_support_full = Decimal("0")
 
         
         # 社会保険料上限を取得
@@ -410,6 +412,15 @@ class PayrollCalculationService:
         employment_insurance_rate = PayrollCalculationService.get_insurance_rate("雇用保険", payment_date)
         employment_insurance = Decimal("0")
         
+        # 子ども・子育て支援金（令和8年4月分以降のみ適用）
+        child_support = Decimal("0")
+        is_child_support_applicable = (
+            (payroll_year == 2026 and payroll_month >= 4) or payroll_year >= 2027
+        )
+        if is_child_support_applicable and calc_social_insurance:
+            child_support = (child_support_full / Decimal("2")).quantize(Decimal("0.01"))
+            print(f"[DEBUG] 子育て支援金: 全額={child_support_full} → 折半額（従業員負担）={child_support}")
+
         # 雇用保険加入対象かどうか確認
         is_employment_insurance_eligible = salary_setting.get("employment_insurance_eligible", True) if salary_setting else True
         
@@ -422,12 +433,13 @@ class PayrollCalculationService:
         elif not employment_insurance_rate and calc_social_insurance:
             errors.append(f"雇用保険料率が見つかりません（年度: {payment_date.year}）")
         
-        # 社会保険計算スキップ時は全て。0にする
+        # 社会保険計算スキップ時は全て0にする
         if not calc_social_insurance:
             health_insurance = Decimal("0")
             care_insurance = Decimal("0")
             pension_insurance = Decimal("0")
             employment_insurance = Decimal("0")
+            child_support = Decimal("0")
         
         # エラーがある場合は、エラーメッセージを返す
         if errors:
@@ -447,7 +459,8 @@ class PayrollCalculationService:
             health_insurance - 
             care_insurance -
             pension_insurance - 
-            employment_insurance
+            employment_insurance -
+            child_support
         )
         
         if calc_income_tax:
@@ -465,6 +478,7 @@ class PayrollCalculationService:
             care_insurance +
             pension_insurance + 
             employment_insurance + 
+            child_support +
             income_tax + 
             resident_tax + 
             other_deduction
@@ -501,6 +515,7 @@ class PayrollCalculationService:
             "care_insurance": care_insurance,
             "pension_insurance": pension_insurance,
             "employment_insurance": employment_insurance,
+            "child_support": child_support,
             "income_tax": income_tax,
             "resident_tax": resident_tax,
             "other_deduction": other_deduction,