Entities land flat, and invites reach strangers

Tenancy stops being a hierarchy nobody's collaborations fit: organization, workspace and project become three labels on one flat table, with no parent_id and no inheritance. Membership removal never cascades, and the API says out loud what a removal did not do. Adding an email that has no didi account now sends an invite from no-reply@didi.sh and attaches the membership when it's redeemed.