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>

backend/app/schemas/hero_banner.py

@@ -103,3 +103,8 @@ class HeroBannerLocalizedResponse(BaseModel):
 
     class Config:
         from_attributes = True
+
+
+# 배너 순서 재정렬 요청
+class BannerReorderRequest(BaseModel):
+    car_ids: list[int]