Closes#72
Upgrades ineligibleReasons from Record<string, string> to a structured
{code, message} type so the UI can render the human-readable reason
inline below the Ineligible badge in both AvailableParticipantsSection
and ParticipantSelectionDialog. API routes surface the message in 400
responses unchanged.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
