feat(API): FCM 채널명 동기화 및 config 일원화 (7채널)

- push_urgent → push_vendor_register (거래처등록)
- push_payment → push_approval_request (결재요청)
- push_income 신규 추가 (입금)
- config/fcm.php에 전체 7개 채널 등록 (기존 2개→7개)
- 서비스 파일 하드코딩을 config() 참조로 전환

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/Services/PushNotificationService.php

@@ -396,12 +396,13 @@ public function notifyGeneral(string $title, string $body, array $data = [], ?in
     private function getChannelForEvent(string $event): string
     {
         return match ($event) {
-            'new_client' => 'push_urgent',
-            'payment' => 'push_payment',
-            'sales_order' => 'push_sales_order',
-            'purchase_order' => 'push_purchase_order',
-            'contract' => 'push_contract',
-            default => 'push_default',
+            'new_client' => config('fcm.channels.vendor_register'),
+            'payment' => config('fcm.channels.approval_request'),
+            'income' => config('fcm.channels.income'),
+            'sales_order' => config('fcm.channels.sales_order'),
+            'purchase_order' => config('fcm.channels.purchase_order'),
+            'contract' => config('fcm.channels.contract'),
+            default => config('fcm.channels.default'),
         };
     }
 }