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/__init__.py

@@ -18,6 +18,7 @@ from .car_specification import CarSpecification
 from .exchange_rate import ExchangeRate, ExchangeRateHistory
 from .cc_package import CCPackage, DEFAULT_CC_PACKAGES
 from .visitor import VisitorLog, VisitorDailyStats, VisitorSession
+from .bulletin import BoardCategory, BoardPost
 
 __all__ = [
     "CarMaker",
@@ -63,4 +64,6 @@ __all__ = [
     "VisitorLog",
     "VisitorDailyStats",
     "VisitorSession",
+    "BoardCategory",
+    "BoardPost",
 ]