feat: 급여 관리 API 구현 (Phase 2: 3.2)

- 마이그레이션: payrolls, payroll_settings 테이블 생성
- 모델: Payroll (상태관리 draft→confirmed→paid), PayrollSetting
- 서비스: PayrollService (4대보험 계산, 급여명세서)
- 컨트롤러: PayrollController + FormRequest 5개
- API 엔드포인트 13개:
  - 급여 CRUD + confirm/pay/payslip
  - 일괄 계산/확정 (calculate, bulk-confirm)
  - 설정 관리 (settings/payroll)
- Swagger 문서: PayrollApi.php
- i18n: error.php, message.php, validation.php 키 추가

app/Http/Requests/V1/Payroll/PayPayrollRequest.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace App\Http\Requests\V1\Payroll;
+
+use Illuminate\Foundation\Http\FormRequest;
+
+class PayPayrollRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    public function rules(): array
+    {
+        return [
+            'withdrawal_id' => ['nullable', 'integer', 'exists:withdrawals,id'],
+        ];
+    }
+
+    public function attributes(): array
+    {
+        return [
+            'withdrawal_id' => __('validation.attributes.withdrawal_id'),
+        ];
+    }
+}