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/schemas/user.py

@@ -42,12 +42,31 @@ class UserResponse(BaseModel):
     referral_code: Optional[str] = None  # User's unique referral code
     email_verified: bool = False
     phone_verified: bool = False
+    promo_preferred_maker: Optional[str] = None
+    promo_preferred_model: Optional[str] = None
+    promo_email_enabled: bool = False
     created_at: datetime
 
     class Config:
         from_attributes = True
 
 
+class PromoPreferenceUpdate(BaseModel):
+    """Schema for updating promo preference"""
+    promo_preferred_maker: Optional[str] = None
+    promo_preferred_model: Optional[str] = None
+    promo_email_enabled: bool = False
+
+
+class PromoPreferenceResponse(BaseModel):
+    promo_preferred_maker: Optional[str] = None
+    promo_preferred_model: Optional[str] = None
+    promo_email_enabled: bool = False
+
+    class Config:
+        from_attributes = True
+
+
 class CarViewResponse(BaseModel):
     id: int
     user_id: int