Request-Rules default approvers and approver picker

## What

Request-Rules UX issues filed the same day; #25040 cites #25039. Complementary (default-to-superusers vs approver picker), not strictly one bug.

## Want

- Request-Rules UX issues filed the same day
- #25040 cites #25039
- Complementary (default-to-superusers vs approver picker), not strictly one bug

## Ideas this came from

- [#25039 Request Rules should have an option to default to superusers](https://github.com/goauthentik/authentik/issues/25039) — Is your feature request related to a problem? Request Rules require a binding to work properly, and I think they should default, to some degree, superusers, or
- [#25040 Improvement to Request Rules approver selection](https://github.com/goauthentik/authentik/issues/25040) — Is your feature request related to a problem? This is an addition to https://github.com/goauthentik/authentik/issues/25039; same existing feature, different req

Start by reading linked issues #25039 and #25040 to understand the two complementary Request-Rules requests and their existing discussion. Then locate the Request-Rules binding and approver-selection entry points in the repository; the work is ready when the desired default-approver and picker behavior is agreed and covered by appropriate tests.

Written by the indexing model from the issue text.