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/bonus/service.py

@@ -139,12 +139,12 @@ class BonusCalculationService:
         # 総支給額
         total_bonus = base_bonus + performance_bonus + other_bonus
         
-        # 社会保険料上限を取得
+        # 社会保険料上限を取得（賞与専用の標準賞与額上限を使用）
         health_insurance_limit = BonusCalculationService.get_insurance_limit(
-            "健康保険標準報酬月額上限", payment_date
+            "健康保険標準賞与額上限", payment_date
         )
         pension_insurance_limit = BonusCalculationService.get_insurance_limit(
-            "厚生年金標準報酬月額上限", payment_date
+            "厚生年金標準賞与額上限", payment_date
         )
         
         # 賞与の標準賞与額（上限適用）
@@ -203,6 +203,34 @@ class BonusCalculationService:
                 total_bonus * Decimal(str(employment_insurance_rate["employee_rate"]))
             ).quantize(Decimal("1"))
         
+        # 子ども・子育て支援金（令和8年4月分以降のみ適用）
+        # 賞与の場合: 標準報酬月額表の淨化率を使い、total_bonus × 子育て率 / 2
+        child_support = Decimal("0")
+        is_child_support_applicable = (
+            (bonus_year == 2026 and bonus_month >= 4) or bonus_year >= 2027
+        )
+        if is_child_support_applicable:
+            with get_connection() as conn:
+                with conn.cursor() as cur:
+                    # 標準報酬月額表から子育て率を取得（最上位等級rowの child_support/monthly_amountで率を計算）
+                    cur.execute(
+                        """
+                        SELECT child_support, monthly_amount
+                        FROM standard_remuneration
+                        WHERE rate_year = %s
+                        AND monthly_amount > 0 AND child_support > 0
+                        ORDER BY monthly_amount DESC
+                        LIMIT 1
+                        """,
+                        (payment_date.year,)
+                    )
+                    sr = cur.fetchone()
+                    if sr and sr["monthly_amount"] and sr["monthly_amount"] > 0:
+                        cs_rate = Decimal(str(sr["child_support"])) / Decimal(str(sr["monthly_amount"]))
+                        # 標準賞与額に封上限なし（法定上賞与には標準賞与額上限なし）
+                        child_support = (total_bonus * cs_rate / Decimal("2")).quantize(Decimal("1"))
+                        print(f"[DEBUG] 賞与子育て支援金: 率={cs_rate}, 賞与総額={total_bonus} → 従業員負担={child_support}")
+        
         # 所得税の計算（賞与特有の計算）
         from ..calculation.service import PayrollCalculationService
         dependents_count = PayrollCalculationService.get_active_dependents_count(employee_id, payment_date)
@@ -214,7 +242,8 @@ class BonusCalculationService:
             health_insurance - 
             care_insurance -
             pension_insurance - 
-            employment_insurance
+            employment_insurance -
+            child_support
         )
         
         income_tax = BonusCalculationService.calculate_bonus_income_tax(
@@ -229,6 +258,7 @@ class BonusCalculationService:
             care_insurance +
             pension_insurance + 
             employment_insurance + 
+            child_support +
             income_tax + 
             other_deduction
         )
@@ -255,6 +285,7 @@ class BonusCalculationService:
             "care_insurance": care_insurance,
             "pension_insurance": pension_insurance,
             "employment_insurance": employment_insurance,
+            "child_support": child_support,
             "income_tax": income_tax,
             "other_deduction": other_deduction,
             "total_deduction": total_deduction,