- user_groups (seeded with the guest and registered groups) and the
users_groups pivot, folded from the PHP v1.1.1 and v2.8.0 migrations
into migration 202610020001_create_user_groups
- models.UserGroup and a User.Groups relation that is never serialized
- classes.UserGroupCodes and classes.HasGroupCode for server-side checks
such as the site-admin group code admin
- additive only: no route, payload or existing column changes; the user
payload still answers "groups":[]
