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(

backend/app/api/hero_banners.py

@@ -1,4 +1,4 @@
-from fastapi import APIRouter, Depends, HTTPException, UploadFile, File, Query, Body
+from fastapi import APIRouter, Depends, HTTPException, UploadFile, File, Query, Body, BackgroundTasks
 from sqlalchemy.orm import Session
 from typing import List, Optional
 import os
@@ -17,6 +17,7 @@ from ..schemas.hero_banner import (
 from .auth import get_current_user
 from ..models import User
 from ..config import get_settings
+from ..services.promo_notification_service import notify_users_for_promo_vehicle
 
 router = APIRouter(prefix="/hero-banners", tags=["hero-banners"])
 
@@ -309,15 +310,16 @@ async def upload_banner_image(
 # ==================== Banner Toggle & Ordering ====================
 
 @router.post("/admin/toggle/{car_id}")
-def toggle_banner(
+async def toggle_banner(
     car_id: int,
+    background_tasks: BackgroundTasks,
     db: Session = Depends(get_db),
     current_user: User = Depends(get_admin_user)
 ):
     """차량의 배너 상태 토글 (Admin)
 
     - HeroBanner 존재 → 삭제
-    - HeroBanner 없음 → 생성
+    - HeroBanner 없음 → 생성 (+ 프로모션 알림 이메일 전송)
     """
     car = db.query(Car).filter(Car.id == car_id).first()
     if not car:
@@ -360,4 +362,8 @@ def toggle_banner(
         car.is_banner = True
         db.commit()
         db.refresh(banner)
+
+        # Send promo notification emails in background
+        background_tasks.add_task(notify_users_for_promo_vehicle, db, car)
+
         return {"car_id": car_id, "is_banner": True, "banner_id": banner.id, "message": "Added to banner"}