Fix banner reorder API with proper request schema

- Create BannerReorderRequest Pydantic model for reorder endpoint
- Update frontend to send car_ids wrapped in object
- Fixes 422 Unprocessable Entity on reorder API

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/src/lib/api.ts

@@ -211,7 +211,7 @@ export const heroBannersApi = {
   },
 
   adminReorderBanners: async (carIds: number[]): Promise<{ message: string; count: number }> => {
-    const { data } = await api.put('/hero-banners/admin/reorder', carIds);
+    const { data } = await api.put('/hero-banners/admin/reorder', { car_ids: carIds });
     return data;
   },