Returns the ticket's conversation thread ({subject, messages[]}, oldest-first) for the AI summary-service. Same shared-secret auth as the config endpoint (class-level attribute; the absolute route below only overrides the class route TEMPLATE, not the auth filter). Hard caps enforced by Infoset.Service.Hubs.Ticket.Service.ITicketThreadService: private notes excluded, last 50 logs, 2000 chars per message, 24000 chars total. A ticket that does not exist in the account — including a foreign-tenant ticket id, since every query is ACCOUNT_ID-scoped — yields 404.