Siblings in the same family share a club membership number. The unique
constraint on (branch_id, membership_id) is dropped. When a receptionist
or admin enters a membership_id that already exists, a confirmation prompt
shows the existing participant's full name so staff can verify it's a
sibling before proceeding. Both the wizard and participant form are updated.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>