feat: add vehicle count column to admin user management

Add a new "Vehicles" column to the admin user management table showing
the count of active vehicles for each user.

Backend changes:
- Add vehicleCount to UserWithAdminStatus type
- Add SQL subquery to count active vehicles (is_active=true, not deleted)
- Add vehicleCount as sortable column option

Frontend changes:
- Add Vehicles column to desktop table (between Tier and Status)
- Add VehicleCountBadge component to mobile user cards
- Update ManagedUser type with vehicleCount field

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

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

frontend/src/features/admin/mobile/AdminUsersMobileScreen.tsx

@@ -81,6 +81,13 @@ const StatusBadge: React.FC<{ active: boolean }> = ({ active }) => (
   </span>
 );
 
+// Vehicle count badge component
+const VehicleCountBadge: React.FC<{ count: number }> = ({ count }) => (
+  <span className="px-2 py-1 rounded-full text-xs font-medium bg-slate-100 text-slate-700">
+    {count} {count === 1 ? 'vehicle' : 'vehicles'}
+  </span>
+);
+
 export const AdminUsersMobileScreen: React.FC = () => {
   const { isAdmin, loading: adminLoading } = useAdminAccess();
 
@@ -453,6 +460,7 @@ export const AdminUsersMobileScreen: React.FC = () => {
                       )}
                       <div className="flex flex-wrap gap-2 mt-2">
                         <TierBadge tier={user.subscriptionTier} />
+                        <VehicleCountBadge count={user.vehicleCount} />
                         <StatusBadge active={!user.deactivatedAt} />
                         {user.isAdmin && (
                           <span className="px-2 py-1 rounded-full text-xs font-medium bg-amber-100 text-amber-700">