feat: 구독/결제 API 확장 (Plan, Subscription, Payment)

- Plan/Subscription/Payment 모델에 상태 상수, 스코프, 헬퍼 메서드 추가
- PlanService, SubscriptionService, PaymentService 생성
- PlanController, SubscriptionController, PaymentController 생성
- FormRequest 9개 생성 (Plan 3개, Subscription 3개, Payment 3개)
- Swagger 문서 3개 생성 (PlanApi, SubscriptionApi, PaymentApi)
- API 라우트 22개 등록 (Plan 7개, Subscription 8개, Payment 7개)
- Pint 코드 스타일 정리

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

@@ -0,0 +1,29 @@
+<?php
+
+namespace App\Http\Requests\V1\Subscription;
+
+use App\Models\Tenants\Subscription;
+use Illuminate\Foundation\Http\FormRequest;
+use Illuminate\Validation\Rule;
+
+class SubscriptionIndexRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    public function rules(): array
+    {
+        return [
+            'status' => ['nullable', 'string', Rule::in(Subscription::STATUSES)],
+            'valid_only' => ['nullable', 'boolean'],
+            'expiring_within' => ['nullable', 'integer', 'min:1', 'max:365'],
+            'start_date' => ['nullable', 'date'],
+            'end_date' => ['nullable', 'date', 'after_or_equal:start_date'],
+            'sort_by' => ['nullable', 'string', 'in:started_at,ended_at,created_at'],
+            'sort_dir' => ['nullable', 'string', 'in:asc,desc'],
+            'per_page' => ['nullable', 'integer', 'min:1', 'max:100'],
+        ];
+    }
+}