Returns the accepted department values with their display labels, in dropdown order. Unauthenticated — the signup form needs it before a token exists. This is the same set POST /v1/users and PATCH /v1/users validate against; a value outside it returns 422 on those routes. Response is flat ({"data": [{"text","value"}]}) rather than JSON:API-enveloped, mirroring the gateway endpoint it replaces.