Backend:
- Use _count.participants for accurate real-time participant count
- Remove reliance on stale participantsCount column
Frontend:
- Show check-in requirement message for non-joined events
- Display "Open chat" button only for joined events
- Add dev-only "View details" button for QR code access during testing
- Improve visual feedback with amber-colored check-in notice
This ensures the participant count reflects actual checked-in users
and prevents unauthorized access to QR codes in production while
maintaining developer convenience in development mode.
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