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/Plan/PlanUpdateRequest.php

@@ -0,0 +1,31 @@
+<?php
+
+namespace App\Http\Requests\V1\Plan;
+
+use App\Models\Tenants\Plan;
+use Illuminate\Foundation\Http\FormRequest;
+use Illuminate\Validation\Rule;
+
+class PlanUpdateRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    public function rules(): array
+    {
+        $planId = $this->route('id');
+
+        return [
+            'name' => ['sometimes', 'required', 'string', 'max:100'],
+            'code' => ['sometimes', 'required', 'string', 'max:50', Rule::unique('plans', 'code')->ignore($planId)],
+            'description' => ['nullable', 'string', 'max:500'],
+            'price' => ['sometimes', 'required', 'numeric', 'min:0'],
+            'billing_cycle' => ['sometimes', 'required', 'string', Rule::in(Plan::BILLING_CYCLES)],
+            'features' => ['nullable', 'array'],
+            'features.*' => ['string', 'max:200'],
+            'is_active' => ['nullable', 'boolean'],
+        ];
+    }
+}