Start a tournament

Seeds the participants, generates the bracket structure and moves the tournament to IN_PROGRESS. ASYNCHRONOUS: this answers 202 and RETURNS BEFORE ANY MATCH EXISTS. The actual matches are generated by a background scheduler chain that runs after the response, and that chain can partially fail - it gives a node ONE attempt and then moves on, so a 202 is not a promise that every match will appear. Poll GET /tournaments/{identifier}/bracket to observe real matches, and use POST /tournaments/{identifier}/regenerate-matches to re-attempt any that are missing. Preconditions: the tournament must be APPROVED and in REGISTRATION_OPEN or REGISTRATION_CLOSED, otherwise 403 error_approval_required or 400. WARNING - when check-in is required, starting REMOVES every approved participant who has not checked in, and the run is then rejected with 400 if fewer than minParticipants remain (the removals are rolled back with it). Requires an API key holding the tournaments lifecycle capability whose owner is a tournament ADMIN - a tournamentStaff MODERATOR is NOT sufficient and receives 403.