Roles overview should show groups and member counts

## What

roles overview: show groups on a role and show member counts.

## Want

- Roles overview: show groups on a role
- Show member counts

## Ideas this came from

- [#19856 Add groups tab to roles view](https://github.com/goauthentik/authentik/issues/19856) — A group tab needs to be added to the roles view. Alongside the user tab:

Start by locating the roles overview entry point and review the linked issues #19856 and #24537 for the intended groups view and member-count behavior. Done means the roles overview shows groups assigned to each role and the number of members, with the existing roles view remaining usable.

Written by the indexing model from the issue text.