fix: improve event check-in UX and participant counting

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.

backend/src/routes/events.js

@@ -19,7 +19,6 @@ router.get('/', authenticate, async (req, res, next) => {
         startDate: true,
         endDate: true,
         worldsdcId: true,
-        participantsCount: true,
         description: true,
         createdAt: true,
         participants: {
@@ -30,6 +29,11 @@ router.get('/', authenticate, async (req, res, next) => {
             joinedAt: true,
           },
         },
+        _count: {
+          select: {
+            participants: true,
+          },
+        },
       },
     });
 
@@ -42,7 +46,7 @@ router.get('/', authenticate, async (req, res, next) => {
       startDate: event.startDate,
       endDate: event.endDate,
       worldsdcId: event.worldsdcId,
-      participantsCount: event.participantsCount,
+      participantsCount: event._count.participants,
       description: event.description,
       createdAt: event.createdAt,
       isJoined: event.participants.length > 0,