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/models/user.py

@@ -37,6 +37,11 @@ class User(Base):
     phone_verified = Column(Boolean, default=False)
     phone_verified_at = Column(DateTime(timezone=True), nullable=True)
 
+    # Promotion preference (프로모션 선호 차종)
+    promo_preferred_maker = Column(String(100), nullable=True)  # 선호 메이커명
+    promo_preferred_model = Column(String(100), nullable=True)  # 선호 모델명
+    promo_email_enabled = Column(Boolean, default=False)  # 프로모션 이메일 수신 동의
+
     # Account withdrawal/deletion
     withdrawal_requested_at = Column(DateTime(timezone=True), nullable=True)  # 탈퇴 요청 시각
     withdrawal_reason = Column(String(500), nullable=True)  # 탈퇴 사유