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/models/settings.py

@@ -63,6 +63,12 @@ class SystemSettings(Base):
     withdrawal_enabled = Column(Boolean, default=True)  # 출금 기능 활성화
     min_withdrawal_usd = Column(Float, default=10.0)  # 최소 출금 금액 (USD)
 
+    # 차량 판매상태 검증 설정
+    car_availability_check_enabled = Column(Boolean, default=True)  # 자동 검증 활성화
+    car_availability_check_hour = Column(Integer, default=6)  # 검증 시간 (0-23, 기본 새벽 6시)
+    car_availability_last_check = Column(DateTime(timezone=True), nullable=True)  # 마지막 검증 시간
+    car_availability_last_result = Column(String(500), nullable=True)  # 마지막 검증 결과 요약
+
     # 타임스탬프
     created_at = Column(DateTime(timezone=True), server_default=func.now())
     updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())