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/page.tsx

@@ -358,17 +358,6 @@ export default function AdminDashboard() {
       <div className="bg-white rounded-xl shadow-sm p-5">
         <h3 className="font-semibold text-gray-800 mb-4">Quick Actions</h3>
         <div className="grid grid-cols-2 md:grid-cols-4 gap-4">
-          <Link
-            href="/admin/hero-banners"
-            className="flex items-center gap-3 p-4 border border-gray-200 rounded-lg hover:border-primary-500 hover:bg-primary-50 transition-colors"
-          >
-            <span className="text-2xl">🖼️</span>
-            <div>
-              <p className="font-medium text-gray-800">Hero Banners</p>
-              <p className="text-xs text-gray-500">Manage slider</p>
-            </div>
-          </Link>
-
           <Link
             href="/admin/cars"
             className="flex items-center gap-3 p-4 border border-gray-200 rounded-lg hover:border-primary-500 hover:bg-primary-50 transition-colors"