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/main.py

@@ -97,6 +97,9 @@ app.include_router(cash.router)
 from app.routers import file_upload
 app.include_router(file_upload.router)
 
+from app.modules.egov.router import router as egov_router
+app.include_router(egov_router)
+
 # ─────────────────────────
 # 給与モジュール (Payroll Module)
 # ─────────────────────────
@@ -118,6 +121,9 @@ app.include_router(payroll_bonus_router)
 from app.payroll.vouchers.router import router as payroll_vouchers_router
 app.include_router(payroll_vouchers_router)
 
+from app.payroll.withholding_slip.router import router as withholding_slip_router
+app.include_router(withholding_slip_router)
+
 
 from fastapi.staticfiles import StaticFiles
 import os