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>

frontend/src/types/index.ts

@@ -106,9 +106,18 @@ export interface User {
   referral_code?: string;
   email_verified: boolean;
   phone_verified: boolean;
+  promo_preferred_maker?: string;
+  promo_preferred_model?: string;
+  promo_email_enabled?: boolean;
   created_at: string;
 }
 
+export interface PromoPreference {
+  promo_preferred_maker?: string;
+  promo_preferred_model?: string;
+  promo_email_enabled: boolean;
+}
+
 export interface CarView {
   id: number;
   user_id: number;