feat: Add bulletin board system

- Add BoardCategory and BoardPost models with multi-language support
- Add bulletin API endpoints (CRUD, notice toggle, pin toggle)
- Add board_enabled setting to control menu visibility
- Create frontend board pages (list, detail, write, edit)
- Create admin board management and category management pages
- Update Header.tsx with conditional Board menu between Inquiry and Contact Us
- Update admin settings with board_enabled toggle
- Add Board menu to admin sidebar

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

backend/app/models/settings.py

@@ -63,6 +63,9 @@ class SystemSettings(Base):
     withdrawal_enabled = Column(Boolean, default=True)  # 출금 기능 활성화
     min_withdrawal_usd = Column(Float, default=10.0)  # 최소 출금 금액 (USD)
 
+    # 게시판 설정
+    board_enabled = Column(Boolean, default=True)  # 게시판 상단 메뉴 표시 여부
+
     # 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)