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/router.py

@@ -0,0 +1,71 @@
+"""
+給与所得の源泉徴収票 APIルーター
+"""
+from fastapi import APIRouter, HTTPException, Query
+from typing import List, Optional
+import logging
+
+from .schemas import WithholdingSlipRequest, WithholdingSlipResponse
+from .service import get_withholding_slips
+from app.core.database import get_connection
+
+logger = logging.getLogger(__name__)
+
+router = APIRouter(prefix="/payroll/withholding-slip", tags=["Payroll - Withholding Slip"])
+
+
+@router.post("/generate", response_model=WithholdingSlipResponse)
+def generate_withholding_slips(request: WithholdingSlipRequest):
+    """
+    指定年度・従業員の給与所得の源泉徴収票データを生成する
+    """
+    if not request.employee_ids:
+        raise HTTPException(status_code=400, detail="従業員IDを指定してください")
+    if request.tax_year < 2000 or request.tax_year > 2099:
+        raise HTTPException(status_code=400, detail="年度が不正です")
+
+    try:
+        slips = get_withholding_slips(request.tax_year, request.employee_ids)
+        return WithholdingSlipResponse(
+            tax_year=request.tax_year,
+            slips=slips,
+            total_count=len(slips)
+        )
+    except Exception as e:
+        logger.error(f"源泉徴収票生成エラー: {e}")
+        raise HTTPException(status_code=500, detail=str(e))
+
+
+@router.get("/employees")
+def get_employees_for_slip():
+    """アクティブな従業員リストを取得（源泉徴収票用）"""
+    try:
+        with get_connection() as conn:
+            with conn.cursor() as cur:
+                cur.execute("""
+                    SELECT employee_id, employee_code, name, name_kana
+                    FROM employees
+                    WHERE is_active = true
+                    ORDER BY employee_code
+                """)
+                rows = cur.fetchall()
+                result = []
+                for row in rows:
+                    if isinstance(row, dict):
+                        result.append({
+                            "id": row["employee_id"],
+                            "code": row["employee_code"],
+                            "name": row["name"],
+                            "name_kana": row.get("name_kana", "")
+                        })
+                    else:
+                        result.append({
+                            "id": row[0],
+                            "code": row[1],
+                            "name": row[2],
+                            "name_kana": row[3] if len(row) > 3 else ""
+                        })
+                return {"employees": result, "total": len(result)}
+    except Exception as e:
+        logger.error(f"従業員リスト取得エラー: {e}")
+        raise HTTPException(status_code=500, detail=str(e))