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/SubmitRequest.php

@@ -0,0 +1,37 @@
+<?php
+
+namespace App\Http\Requests\Approval;
+
+use App\Models\Tenants\ApprovalLine;
+use Illuminate\Foundation\Http\FormRequest;
+
+class SubmitRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    public function rules(): array
+    {
+        $stepTypes = implode(',', ApprovalLine::STEP_TYPES);
+
+        return [
+            'steps' => 'required|array|min:1',
+            'steps.*.type' => "required|string|in:{$stepTypes}",
+            'steps.*.user_id' => 'required|integer|exists:users,id',
+        ];
+    }
+
+    public function messages(): array
+    {
+        return [
+            'steps.required' => __('error.approval.steps_required'),
+            'steps.min' => __('validation.min.array', ['attribute' => '결재단계', 'min' => 1]),
+            'steps.*.type.required' => __('validation.required', ['attribute' => '단계유형']),
+            'steps.*.type.in' => __('validation.in', ['attribute' => '단계유형']),
+            'steps.*.user_id.required' => __('validation.required', ['attribute' => '결재자']),
+            'steps.*.user_id.exists' => __('validation.exists', ['attribute' => '결재자']),
+        ];
+    }
+}