feat: Phase 8 SaaS 확장 - 구독관리/결제내역 API 추가

- 사용량 조회 API (GET /subscriptions/usage)
- 데이터 내보내기 API (POST/GET /subscriptions/export)
- 결제 명세서 API (GET /payments/{id}/statement)
- DataExport 모델 및 마이그레이션 추가

app/Http/Requests/V1/Subscription/ExportStoreRequest.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace App\Http\Requests\V1\Subscription;
+
+use App\Models\Tenants\DataExport;
+use Illuminate\Foundation\Http\FormRequest;
+use Illuminate\Validation\Rule;
+
+class ExportStoreRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    public function rules(): array
+    {
+        return [
+            'export_type' => ['required', 'string', Rule::in(DataExport::TYPES)],
+            'options' => ['nullable', 'array'],
+            'options.format' => ['nullable', 'string', Rule::in(['xlsx', 'csv', 'json'])],
+            'options.include_deleted' => ['nullable', 'boolean'],
+        ];
+    }
+
+    public function attributes(): array
+    {
+        return [
+            'export_type' => __('field.export_type'),
+            'options' => __('field.options'),
+        ];
+    }
+}