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/schemas/settings.py

@@ -32,6 +32,9 @@ class SystemSettingsUpdate(BaseModel):
     withdrawal_enabled: Optional[bool] = None
     min_withdrawal_usd: Optional[float] = None
 
+    # 게시판 설정
+    board_enabled: Optional[bool] = None
+
     # 차량 판매상태 검증 설정
     car_availability_check_enabled: Optional[bool] = None
     car_availability_check_hour: Optional[int] = None
@@ -67,6 +70,9 @@ class SystemSettingsResponse(BaseModel):
     withdrawal_enabled: bool = True
     min_withdrawal_usd: float = 10.0
 
+    # 게시판 설정
+    board_enabled: bool = True
+
     # 차량 판매상태 검증 설정
     car_availability_check_enabled: bool = True
     car_availability_check_hour: int = 6