Added: POST /profiles/api/v1/batch — many public profiles in one request
(#111). A missing profile comes back in `missing`, never as a 404; the
PROFILES_BATCH_MAX_IDS ceiling refuses rather than truncates.

Fixed: the user.registered display_name hint was read and thrown away
(#149). It pre-fills, it never assigns — a name the human set (or
deliberately cleared past onboarding) is never overwritten by a
redelivery.