Add configurable CC cost for banner vehicle PDF/description view

- Add cc_per_banner_view setting to system_settings (default 0.1 CC)
- Update car detail page to use dynamic CC value from settings
- Add CC per Banner View field in admin settings page
- Replace hardcoded 0.1 CC with configurable value

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

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

backend/app/models/settings.py

@@ -21,6 +21,7 @@ class SystemSettings(Base):
     cc_per_view = Column(Integer, default=1)   # 차량 상세 조회 시 1 CC
     cc_signup_bonus = Column(Integer, default=3)  # 신규 가입 시 3 CC
     cars_per_cc = Column(Integer, default=3)  # 1 CC당 추천 차량 수 (기본 3대)
+    cc_per_banner_view = Column(Float, default=0.1)  # 배너 차량 PDF/상세 보기 비용 (기본 0.1 CC)
 
     # 캐시 TTL (시간)
     cache_ttl_hours = Column(Integer, default=2)