feat: [approval] 전자결재 모듈 API 구현

- 마이그레이션 4개 (approval_forms, approval_lines, approvals, approval_steps)
- 모델 4개 (ApprovalForm, ApprovalLine, Approval, ApprovalStep)
- ApprovalService 비즈니스 로직 (양식/결재선 CRUD, 기안함/결재함/참조함, 결재 액션)
- 컨트롤러 3개 (ApprovalFormController, ApprovalLineController, ApprovalController)
- FormRequest 13개 (양식/결재선/문서 검증)
- Swagger 문서 3개 (26개 엔드포인트)
- i18n 메시지/에러 키 추가
- 라우트 26개 등록

app/Http/Requests/Approval/ReferenceIndexRequest.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace App\Http\Requests\Approval;
+
+use Illuminate\Foundation\Http\FormRequest;
+
+class ReferenceIndexRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    public function rules(): array
+    {
+        return [
+            'is_read' => 'nullable|boolean',
+            'sort_by' => 'nullable|string|in:created_at,drafted_at,completed_at,title',
+            'sort_dir' => 'nullable|string|in:asc,desc',
+            'per_page' => 'nullable|integer|min:1|max:100',
+            'page' => 'nullable|integer|min:1',
+        ];
+    }
+}