Circle ownership can be assigned to non-acting user/invalid email address

Hello,

I experienced the following issue/bug (there's no template so I'm hoping I'm providing all relevant information here):

Steps to reproduce

1. On https://framagenda.org, create an account and create a circle
2. On your own Nextcloud instance (e.g. https://cloud.keunes.com), figure out your federated ID: keunes@cloud.keunes.com
3. On framagenda.org, open the created circle and click 'Add members'
4. Enter the federated ID: keunes@cloud.keunes.com and click on the button "Add to [circle name]"
5. Click on the three dots of the added account/member and then 'Promote to owner)

The problem
The federated user ID seems to be interpreted as an email address. (I'm not sure if federated users can be added to circles.) However, it is not a valid email address and cannot receive emails with invites. At this point, the account that cannot be accessed is marked as owner. The circle can no longer be deleted by anyone (but the admins, I suppose).

Proposed solution
Only allow ownership to be assigned to users on the instance of the circle and to federated users, not to email accounts.

Start by reproducing the ownership-promotion flow described on framagenda.org with a federated ID and an email account. Trace how circle members are classified when promoted to owners, then verify that ownership is restricted as proposed and that a circle with an invalid owner remains manageable. No files or tests are named in the issue.