Adds finalist.unenroll(projectId, roundId) which deletes the
FinalistConfirmation (cascading AttendingMember/FlightDetail/
VisaApplication/MemberLunchPick) and the LIVE_FINAL ProjectRoundState,
then logs a FINALIST_UNENROLL audit entry. Safe no-op when no rows exist.
Tests cover ADMIN_CONFIRM enrolled teardown and the no-rows path.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
