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GitHub User: astronaute
It is the user that create/read/update/delete the tenant, not the other way around, so he can have multiple tenants. And in theory, a user can exist without any tenant if you desire so. You just need to implement a role based authorization system in your app.
App wide super-admin can be implemented as a kind of super user that has access to all tenants, but it's better to have it's own class so you can also assign different roles to it.