fix: correct dict unpacking in admin_get_users to resolve validation error

backend/app/api.py

@@ -469,12 +469,12 @@ async def admin_get_users(
     users = db.query(User).all()
     result = []
     for u in users:
-        is_vip, expire_at = check_vip_status(u.id, db)
+        vip_info = check_vip_status(u.id, db)
         result.append(AdminUserItem(
             user_id=u.id,
             username=u.username,
-            is_vip=is_vip,
-            vip_expire_at=expire_at,
+            is_vip=vip_info["is_vip"],
+            vip_expire_at=vip_info["vip_expire_at"],
             created_at=u.created_at.strftime('%Y-%m-%d %H:%M:%S') if u.created_at else ""
         ))
     return result