POST /api/forge/users/{name}/admin reads the account and, when it is not
already a site admin, sets `admin` with admin_edit_user. It never
creates one: a human's account is made by their first authelia login,
so a missing one answers 404, saying the user has not logged in via SSO
yet. An existing admin is a success with nothing sent.
The edit carries `admin` alone. repo_creation_lockdown's login_name +
source_id = 0 would turn an SSO-made account into a local one: in
Forgejo 16 a source_id sets the login type.
An agent's name is refused, and so is any name when the roster can't be
read: a site admin ignores max_repo_creation, the lockdown that keeps an
agent's token from creating a repo and self-merging in it.
Refs #3782
