feat: auto-register walk-in attendees during check-in

- Automatically register users who scan QR code but aren't pre-registered
- Mark walk-in registrations with wasWalkIn flag
- Show different success messages for walk-ins vs pre-registered attendees
- Check in walk-ins immediately upon registration
- Remove 'not registered' error - now handles walk-ins gracefully
- Update both QR scan and manual check-in to support walk-ins

server/src/controllers/eventController.ts

@@ -582,20 +582,12 @@ export const eventController = {
       // Find or create the attendee user
       const attendeeUserId = await findOrCreateUser(clerkUserId);
 
-      // Check in the attendee
+      // Check in the attendee (will auto-register if not registered)
       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({