As admin view list of users occupying a license seat

### Please agree to the following

- [X] I have searched [existing issues](https://github.com/cryptomator/hub/issues?q=) for duplicates
- [X] I agree to follow this project's [Code of Conduct](https://github.com/cryptomator/hub/blob/develop/.github/CODE_OF_CONDUCT.md)

### Summary

As an admin, i want to view list of users occupying a seat. Optionally, which vaults they are part of.

### Motivation

If the license is exceeded, i want to be able to check, who is assigned to which vault. With that information, i can decide wether i upgrade the license or remove users from of a vault.

### Considered Alternatives

_No response_

### Anything else?

Since version 1.3.3 this case will happen less frequently, because only due to an license update (reducing seats) or adding members to group it can happen.

The issue names no files, tests, or entry points. Start by locating the existing admin license-seat view and the user-to-vault membership data, then determine how to show users occupying seats and optionally their vaults; done means an administrator can identify those assignments when seats are exceeded.

Written by the indexing model from the issue text.