Add door check-in flow for Main Tickets

Lets staff redeem a registration's Main Tickets by quantity at the door
(via the Payment/registration flow) instead of scanning each QR code,
and automatically emails/WhatsApps a check-in confirmation to the
attendee. Also routes the At The Door "Open" button and post-payment
flow dynamically: paid registrations jump straight to Check-In instead
of a forced ticket print, since tickets are already sent automatically.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/src/utils/waMessages.js

@@ -107,6 +107,26 @@ function buildWAPayment(payment) {
   ].join('\n');
 }
 
+// ─── Check-in confirmation (Main Tickets, door check-in) ───────────────────────
+
+function buildWACheckIn(ticket, qtyRedeemed, totalRedeemed, remaining) {
+  const org        = getOrg();
+  const eventTitle = ticket.event?.title || 'the event';
+  const name       = ticket.user?.name || 'there';
+  const fullyCheckedIn = remaining <= 0;
+
+  return [
+    `✅ *Checked In*`,
+    '',
+    `Hi ${name},`,
+    '',
+    `*${qtyRedeemed}* checked in for *${eventTitle}*.`,
+    `*${totalRedeemed} of ${ticket.quantity || 1}* checked in${fullyCheckedIn ? '' : `, *${remaining}* remaining`}.`,
+    '',
+    `_${org.name}_ | ${org.url}`,
+  ].join('\n');
+}
+
 // ─── Login notification ───────────────────────────────────────────────────────
 
 function buildWALogin({ name, when, location, userAgent }) {
@@ -255,4 +275,5 @@ module.exports = {
   buildWAWelcome,
   buildWAAccountClosed,
   buildWATicketCaption,
+  buildWACheckIn,
 };