feat: [fcm] Admin FCM API 추가 - MNG에서 API 호출로 FCM 발송

- AdminFcmController, AdminFcmService 추가
- FormRequest 검증 (AdminFcmSendRequest 등)
- Swagger 문서 추가 (AdminFcmApi.php)
- ApiKeyMiddleware: admin/fcm/* 화이트리스트 추가
- FCM 에러 메시지 i18n 추가

app/Http/Requests/Admin/SendFcmRequest.php

@@ -0,0 +1,37 @@
+<?php
+
+namespace App\Http\Requests\Admin;
+
+use Illuminate\Foundation\Http\FormRequest;
+
+class SendFcmRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    public function rules(): array
+    {
+        return [
+            'title' => 'required|string|max:255',
+            'body' => 'required|string|max:1000',
+            'tenant_id' => 'nullable|integer|exists:tenants,id',
+            'user_id' => 'nullable|integer|exists:users,id',
+            'platform' => 'nullable|string|in:android,ios,web',
+            'channel_id' => 'nullable|string|max:50',
+            'type' => 'nullable|string|max:50',
+            'url' => 'nullable|string|max:500',
+            'sound_key' => 'nullable|string|max:50',
+        ];
+    }
+
+    public function messages(): array
+    {
+        return [
+            'title.required' => __('validation.required', ['attribute' => '제목']),
+            'body.required' => __('validation.required', ['attribute' => '내용']),
+            'platform.in' => __('validation.in', ['attribute' => '플랫폼']),
+        ];
+    }
+}