feat: add user count display for each role

- Add Users column showing number of users assigned to each role
- Load user counts efficiently in single query to avoid N+1
- Similar implementation to membership fee types member count

lib/mv_web/live/role_live/index.html.heex

@@ -49,6 +49,10 @@
       <% end %>
     </:col>
 
+    <:col :let={role} label={gettext("Users")}>
+      <span class="badge badge-ghost">{get_user_count(role, @user_counts)}</span>
+    </:col>
+
     <:action :let={role}>
       <div class="sr-only">
         <.link navigate={~p"/admin/roles/#{role}"}>{gettext("Show")}</.link>