feat: Users API, ownership transfer and removal of the legacy role string

features/user_roles.feature

@@ -1,5 +1,6 @@
 Feature: Users have a role, owner or admin, and only the owner can invite
   Superseded in part by roles_permissions_enforcement: the owner and admin 403 clauses now answer 403 "missing_permission" with the header X-Missing-Permission, decided by the caller's role permissions. An Admin can now invite and write sites, and a role without the needed permission is refused on routes that were open to every verified user.
+  Superseded in part by roles_permissions_users_api: a user has one stored role, role_id, and the user.Role type with the "owner" and "admin" strings no longer exists. AcceptInvite creates the user with the role of the invite (Writer by default, set by the inviter), not Admin. GET /v1/auth/me carries no top-level role (access.role only), the users list shows role as a RoleRef {id, name, built_in, locked} or null, and PUT /v1/user/{id}/role and POST /v1/site/transfer-ownership now exist, so "no endpoint transfers a role" no longer holds.
 
   As the person who registered the workspace
   I want my account to be the owner and everyone I invite to be an admin