feat: implement QR code check-in system for event attendees

- Add UserQRCode component to display unique QR code on user profile
- Add QRCodeScanner component for mobile camera-based scanning
- Create EventCheckInPage for organizers to scan attendee QR codes
- Add backend endpoint POST /api/events/:id/checkin for check-in
- Add check-in button to EventDetailModal for organizers
- Install qrcode.react and html5-qrcode dependencies
- Implement real-time attendee list updates
- Add organizer permission validation
- Prevent duplicate check-ins
- Mobile-optimized with back camera support

server/src/controllers/eventController.ts

@@ -536,4 +536,81 @@ export const eventController = {
       });
     }
   },
+
+  // QR Code Check-in
+  async checkInAttendee(req: AuthRequest, res: Response) {
+    try {
+      const organizerClerkId = req.userId;
+      if (!organizerClerkId) {
+        return res.status(401).json({
+          code: 'UNAUTHORIZED',
+          message: 'User not authenticated',
+          timestamp: new Date().toISOString(),
+        });
+      }
+
+      const organizerId = await findOrCreateUser(organizerClerkId);
+      const { id } = req.params;
+      const { clerkUserId } = req.body;
+
+      if (!clerkUserId) {
+        return res.status(400).json({
+          code: 'VALIDATION_ERROR',
+          message: 'clerkUserId is required',
+          timestamp: new Date().toISOString(),
+        });
+      }
+
+      // Verify event exists and user is organizer
+      const event = await eventService.getEventById(id);
+      if (!event) {
+        return res.status(404).json({
+          code: 'EVENT_NOT_FOUND',
+          message: 'Event not found',
+          timestamp: new Date().toISOString(),
+        });
+      }
+
+      if (event.organizerId !== organizerId) {
+        return res.status(403).json({
+          code: 'FORBIDDEN',
+          message: 'Only the event organizer can check in attendees',
+          timestamp: new Date().toISOString(),
+        });
+      }
+
+      // Find or create the attendee user
+      const attendeeUserId = await findOrCreateUser(clerkUserId);
+
+      // Check in the attendee
+      const result = await eventService.checkInAttendee(id, attendeeUserId);
+
+      res.json(result);
+    } catch (error: any) {
+      console.error('Error checking in attendee:', error);
+      
+      if (error.message.includes('not registered')) {
+        return res.status(404).json({
+          code: 'NOT_REGISTERED',
+          message: error.message,
+          timestamp: new Date().toISOString(),
+        });
+      }
+
+      if (error.message.includes('already checked in')) {
+        return res.status(409).json({
+          code: 'ALREADY_CHECKED_IN',
+          message: error.message,
+          timestamp: new Date().toISOString(),
+        });
+      }
+
+      res.status(500).json({
+        code: 'CHECKIN_ERROR',
+        message: 'Failed to check in attendee',
+        details: error.message,
+        timestamp: new Date().toISOString(),
+      });
+    }
+  },
 };