切换激活空间

Memberships lists every workspace the user can authenticate into, along with their role in each. Always populated (length 1 for users who only belong to their home workspace) so frontends can render a workspace switcher without a follow-up request. Serialised without omitempty so the field is always present as a JSON array (possibly empty) — the "always populated" contract relies on the server side guaranteeing a non-nil slice.