feat: 인스타 슬레이트 패키지 다운로드 버튼

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/api.js

@@ -548,6 +548,8 @@ export function getInstaAssetUrl(slateId, page) {
   return `/api/insta/slates/${slateId}/assets/${page}`;
 }
 
+export const instaPackageUrl = (slateId) => `/api/insta/slates/${slateId}/package`;
+
 export function getInstaTask(taskId) {
   return apiGet(`/api/insta/tasks/${encodeURIComponent(taskId)}`);
 }