feat: 給与所得の源泉徴収票機能を追加

- 新規モジュール: backend/app/payroll/withholding_slip/
  - schemas.py: 源泉徴収票データモデル
  - service.py: 年間集計・給与所得控除・控除額計算ロジック
  - router.py: POST /payroll/withholding-slip/generate
- frontend/withholding-slip.html: 票面プレビュー・印刷（3部）ページ
- frontend/payroll.html: 源泉徴収票メニューカード追加
- backend/app/main.py: ルーター登録

backend/app/payroll/withholding_slip/schemas.py

@@ -0,0 +1,95 @@
+"""
+給与所得の源泉徴収票 スキーマ定義
+"""
+from pydantic import BaseModel
+from typing import Optional, List
+from decimal import Decimal
+
+
+class DependentInfo(BaseModel):
+    """扶養親族情報"""
+    name: str
+    name_kana: Optional[str] = None
+    relationship: Optional[str] = None
+    birth_date: Optional[str] = None
+    is_spouse: bool = False
+    is_disabled: bool = False
+    income_amount: Decimal = Decimal("0")
+    my_number: Optional[str] = None
+    deduction_type: Optional[str] = None  # 一般/特定/老人(同居)/老人(その他)
+    deduction_amount: Decimal = Decimal("0")
+
+
+class WithholdingSlipData(BaseModel):
+    """源泉徴収票データ"""
+    # 従業員情報
+    employee_id: int
+    employee_code: str
+    employee_name: str
+    employee_name_kana: Optional[str] = None
+    employee_address: Optional[str] = None
+    employee_birth_date: Optional[str] = None
+    my_number: Optional[str] = None
+
+    # 対象年
+    tax_year: int
+
+    # 支払金額
+    total_payment: Decimal = Decimal("0")           # 年間総支給額（通勤手当含む）
+    total_payment_excl_commute: Decimal = Decimal("0")  # 通勤手当を除く支払金額（源泉徴収票記載用）
+
+    # 給与所得控除後の金額
+    kyuyo_shotoku: Decimal = Decimal("0")
+
+    # 社会保険料等の金額
+    social_insurance_total: Decimal = Decimal("0")
+    health_insurance_total: Decimal = Decimal("0")
+    care_insurance_total: Decimal = Decimal("0")
+    pension_total: Decimal = Decimal("0")
+    employment_insurance_total: Decimal = Decimal("0")
+
+    # 基礎控除
+    basic_deduction: Decimal = Decimal("0")
+
+    # 配偶者控除
+    has_spouse_deduction: bool = False
+    spouse_deduction_amount: Decimal = Decimal("0")
+    spouse_name: Optional[str] = None
+    spouse_name_kana: Optional[str] = None
+    spouse_income: Decimal = Decimal("0")
+    spouse_deduction_type: Optional[str] = None  # 配偶者控除/配偶者特別控除
+
+    # 扶養親族
+    dependents_over16: List[DependentInfo] = []    # 16歳以上控除対象扶養親族
+    dependents_under16: List[DependentInfo] = []   # 16歳未満扶養親族
+    dependent_deduction_total: Decimal = Decimal("0")
+
+    # 所得控除の額の合計額
+    total_deduction_amount: Decimal = Decimal("0")
+
+    # 源泉徴収税額
+    income_tax_withheld: Decimal = Decimal("0")
+
+    # 月数内訳
+    months_with_salary: int = 0
+    months_with_bonus: int = 0
+
+    # 摘要
+    remarks: Optional[str] = None
+
+
+class WithholdingSlipRequest(BaseModel):
+    """源泉徴収票生成リクエスト"""
+    tax_year: int
+    employee_ids: List[int]
+    company_name: Optional[str] = None
+    company_address: Optional[str] = None
+    company_phone: Optional[str] = None
+    company_my_number: Optional[str] = None
+
+
+class WithholdingSlipResponse(BaseModel):
+    """源泉徴収票レスポンス"""
+    tax_year: int
+    slips: List[WithholdingSlipData]
+    total_count: int