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>

backend/src/features/user-profile/data/user-profile.repository.ts

@@ -176,6 +176,7 @@ export class UserProfileRepository {
       ...this.mapRowToUserProfile(row),
       isAdmin: !!row.admin_auth0_sub,
       adminRole: row.admin_role || null,
+      vehicleCount: parseInt(row.vehicle_count, 10) || 0,
     };
   }
 
@@ -196,6 +197,7 @@ export class UserProfileRepository {
       createdAt: 'up.created_at',
       displayName: 'up.display_name',
       subscriptionTier: 'up.subscription_tier',
+      vehicleCount: 'vehicle_count',
     };
     const sortColumn = sortColumnMap[sortBy] || 'up.created_at';
 
@@ -234,14 +236,18 @@ export class UserProfileRepository {
       ${whereClause}
     `;
 
-    // Data query with admin status join
+    // Data query with admin status join and vehicle count
     const dataQuery = `
       SELECT
         up.id, up.auth0_sub, up.email, up.display_name, up.notification_email,
         up.subscription_tier, up.email_verified, up.onboarding_completed_at,
         up.deactivated_at, up.deactivated_by, up.created_at, up.updated_at,
         au.auth0_sub as admin_auth0_sub,
-        au.role as admin_role
+        au.role as admin_role,
+        (SELECT COUNT(*) FROM vehicles v
+         WHERE v.user_id = up.auth0_sub
+         AND v.is_active = true
+         AND v.deleted_at IS NULL) as vehicle_count
       FROM user_profiles up
       LEFT JOIN admin_users au ON up.auth0_sub = au.auth0_sub AND au.revoked_at IS NULL
       ${whereClause}
@@ -275,7 +281,11 @@ export class UserProfileRepository {
         up.subscription_tier, up.email_verified, up.onboarding_completed_at,
         up.deactivated_at, up.deactivated_by, up.created_at, up.updated_at,
         au.auth0_sub as admin_auth0_sub,
-        au.role as admin_role
+        au.role as admin_role,
+        (SELECT COUNT(*) FROM vehicles v
+         WHERE v.user_id = up.auth0_sub
+         AND v.is_active = true
+         AND v.deleted_at IS NULL) as vehicle_count
       FROM user_profiles up
       LEFT JOIN admin_users au ON up.auth0_sub = au.auth0_sub AND au.revoked_at IS NULL
       WHERE up.auth0_sub = $1