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/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"}