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/pages/admin/AdminUsersPage.tsx

@@ -420,6 +420,7 @@ export const AdminUsersPage: React.FC = () => {
                     <TableCell>Email</TableCell>
                     <TableCell>Display Name</TableCell>
                     <TableCell>Tier</TableCell>
+                    <TableCell>Vehicles</TableCell>
                     <TableCell>Status</TableCell>
                     <TableCell>Admin</TableCell>
                     <TableCell>Created</TableCell>
@@ -450,6 +451,7 @@ export const AdminUsersPage: React.FC = () => {
                           </Select>
                         </FormControl>
                       </TableCell>
+                      <TableCell>{user.vehicleCount}</TableCell>
                       <TableCell>
                         <Chip
                           label={user.deactivatedAt ? 'Deactivated' : 'Active'}