API notif dan penambahan package firebase-admin

Add:
- src/app/api/mobile/notifications/
- src/lib/firebase-admin.ts

### No Issue

src/lib/firebase-admin.ts

@@ -0,0 +1,24 @@
+// lib/firebase-admin.ts
+import { cert, getApp, getApps, initializeApp } from 'firebase-admin/app';
+import { getMessaging } from 'firebase-admin/messaging';
+
+// Ambil dari environment
+const serviceAccount = {
+  projectId: process.env.FIREBASE_ADMIN_PROJECT_ID,
+  clientEmail: process.env.FIREBASE_ADMIN_CLIENT_EMAIL,
+  privateKey: process.env.FIREBASE_ADMIN_PRIVATE_KEY?.replace(/\\n/g, '\n'),
+};
+
+if (!serviceAccount.projectId || !serviceAccount.clientEmail || !serviceAccount.privateKey) {
+  throw new Error('Firebase Admin credentials are missing in environment variables');
+}
+
+// Inisialisasi hanya sekali
+const app = !getApps().length
+  ? initializeApp({
+      credential: cert(serviceAccount),
+      projectId: serviceAccount.projectId,
+    })
+  : getApp();
+
+export const adminMessaging = getMessaging(app);