feat: Add promo preference survey on main page

- Add promo preference fields to User model (promo_preferred_maker,
  promo_preferred_model, promo_email_enabled)
- Create API endpoints for getting/updating promo preferences
- Create PromoPreference component with maker/model selection
- Show login prompt for non-logged-in users when interacting
- Add promo notification service to send emails when matching vehicles
  are added to promotion
- Add multi-language translations (en, mn, ru, ko)

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

backend/app/api/auth.py

@@ -10,6 +10,7 @@ from ..database import get_db
 from ..models import User, SystemSettings, ReferralReward
 from ..models.user import generate_referral_code
 from ..schemas import UserCreate, UserUpdate, UserResponse, Token
+from ..schemas.user import PromoPreferenceUpdate, PromoPreferenceResponse
 from ..config import get_settings
 
 router = APIRouter(prefix="/auth", tags=["auth"])
@@ -277,6 +278,28 @@ def update_me(
     return current_user
 
 
+@router.get("/me/promo-preference", response_model=PromoPreferenceResponse)
+def get_promo_preference(current_user: User = Depends(get_current_user)):
+    """프로모션 선호 차종 조회"""
+    return current_user
+
+
+@router.put("/me/promo-preference", response_model=PromoPreferenceResponse)
+def update_promo_preference(
+    promo_update: PromoPreferenceUpdate,
+    current_user: User = Depends(get_current_user),
+    db: Session = Depends(get_db)
+):
+    """프로모션 선호 차종 설정"""
+    current_user.promo_preferred_maker = promo_update.promo_preferred_maker
+    current_user.promo_preferred_model = promo_update.promo_preferred_model
+    current_user.promo_email_enabled = promo_update.promo_email_enabled
+
+    db.commit()
+    db.refresh(current_user)
+    return current_user
+
+
 # Admin User Management Endpoints
 @router.get("/admin/users")
 def admin_get_users(