feat: Add car availability check feature

- Add daily scheduled check for Carmodoo car availability
- Add manual trigger button in admin settings
- Mark sold cars as soldout=True automatically
- Add settings for check time and enable/disable toggle
- Display check status and statistics in admin UI

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

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

backend/app/schemas/settings.py

@@ -32,6 +32,10 @@ class SystemSettingsUpdate(BaseModel):
     withdrawal_enabled: Optional[bool] = None
     min_withdrawal_usd: Optional[float] = None
 
+    # 차량 판매상태 검증 설정
+    car_availability_check_enabled: Optional[bool] = None
+    car_availability_check_hour: Optional[int] = None
+
 
 class SystemSettingsResponse(BaseModel):
     """시스템 설정 응답 스키마"""
@@ -63,6 +67,12 @@ class SystemSettingsResponse(BaseModel):
     withdrawal_enabled: bool = True
     min_withdrawal_usd: float = 10.0
 
+    # 차량 판매상태 검증 설정
+    car_availability_check_enabled: bool = True
+    car_availability_check_hour: int = 6
+    car_availability_last_check: Optional[datetime] = None
+    car_availability_last_result: Optional[str] = None
+
     created_at: Optional[datetime] = None
     updated_at: Optional[datetime] = None