feat: Add banner toggle and soldout tracking to Cars page

- Add is_banner, soldout fields to Car model
- Add banner toggle API (POST /hero-banners/admin/toggle/{car_id})
- Add soldout APIs (POST/DELETE /cars/{car_id}/soldout)
- Add nightly soldout checker in agent (runs at 3:00 AM)
- Update Local Cars UI with banner checkbox and status column
- Remove hero-banners admin page (functionality moved to Cars page)
- Banner cars sorted to top with purple background
- Soldout cars displayed with gray overlay

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/app/admin/layout.tsx

@@ -8,7 +8,6 @@ import { useAuthStore } from '@/lib/store';
 const menuItems = [
   { href: '/admin', label: 'Dashboard', icon: '📊' },
   { href: '/admin/visitor-stats', label: 'Visitor Stats', icon: '👁️' },
-  { href: '/admin/hero-banners', label: 'Hero Banners', icon: '🖼️' },
   { href: '/admin/cars', label: 'Cars', icon: '🚗' },
   { href: '/admin/vehicle-requests', label: 'Vehicle Requests', icon: '📋' },
   { href: '/admin/purchased', label: 'Purchased Vehicles', icon: '📦' },