Fix hardcoded localhost:8000 URLs

- Add backend/utils.py for URL management
- Update serializers to use normalize_file_url()
- Update views to use URL utils from env vars
- Fix frontend components to use NEXT_PUBLIC_API_URL
- Add new env vars: DJANGO_BACKEND_URL, DJANGO_MEDIA_BASE_URL
- Replace all hardcoded localhost:8000 with configurable URLs

frontend/linktree-frontend/src/app/[username]/page.tsx

@@ -84,7 +84,7 @@ export default function UserPage({
 
       // Определяем API URL в зависимости от окружения
       const API = typeof window !== 'undefined' 
-        ? (process.env.NEXT_PUBLIC_API_URL || 'http://localhost:8000')  // клиент
+        ? (process.env.NEXT_PUBLIC_API_URL || 'https://links.shareon.kr')  // клиент
         : 'http://web:8000'  // сервер в Docker
 
       console.log('Loading data for user:', usernameValue)