Super admin and financial admin can select up to 2 confirmed payments from
a participant's profile and refund them in a single atomic transaction.
If a refunded payment is linked to a program enrollment, the enrollment is
automatically cancelled and the group seat is freed.

Changes:
- RefundService: creates outbound refund record, marks original as refunded,
  reverses invoice paid_amount — all within the caller's DB::transaction
- ParticipantShow: checkbox selection on confirmed inbound payments, preview
  modal showing impact (including enrollment cancellation warning), reason
  field, atomic commit via DB::transaction wrapping both domains
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>