Close tournament registration

Moves a REGISTRATION_OPEN tournament to REGISTRATION_CLOSED. Takes no request body. Any other status answers 400. This is REVERSIBLE - registration can be re-opened from REGISTRATION_CLOSED. Unlike opening, closing emits NO webhook and does not change the tournament's public listing, so a webhook consumer will not observe this transition; poll the tournament if you need to react to 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.