Speed up payment/refund processing on the supervisor dashboard

Reconciling a Yoco payment and sending payment links blocked the HTTP
response on ticket-PDF generation and email/WhatsApp sends; they now
run in the background like the other payment flows already did.
Registration/payment option loops (pricing, stock checks, ticket
generation) now resolve concurrently instead of sequentially. The
Payments page dropped a per-registration N+1 fetch and now refreshes
its lists in parallel after each action. Added missing indexes for
dashboard stats and donation-leg lookups, and made GET
/api/registrations optionally paginated.

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

backend/src/controllers/paymentController.js

@@ -1057,20 +1057,22 @@ const sendPaymentLink = async (req, res) => {
     const { user, event } = registration;
     const message = `Hi ${user.name || ''}, here's your payment link for ${event?.title || 'your registration'}: ${redirectUrl}`;
 
+    // Validation is synchronous (fast, no network); the actual send is backgrounded since
+    // SMTP/WAWP round trips shouldn't block this request.
     if (channel === 'email') {
       if (!user.email) { res.status(400); throw new Error('This user has no email address on file'); }
       const { sendMail } = require('../utils/email');
-      await sendMail({
+      sendMail({
         to: user.email,
         subject: `Payment link — ${event?.title || 'Registration'}`,
         text: message,
         html: `<p>Hi ${user.name || ''},</p><p>Here's your payment link for <strong>${event?.title || 'your registration'}</strong>:</p><p><a href="${redirectUrl}">${redirectUrl}</a></p>`
-      });
+      }).catch(e => console.error('Failed to send payment link email:', e));
     } else {
       const { isValidZAPhone } = require('../utils/whatsapp');
       if (!isValidZAPhone(user.phoneNumber)) { res.status(400); throw new Error('This user has no valid WhatsApp number on file'); }
       const { waTextAny } = require('../utils/notify');
-      await waTextAny(user, message);
+      waTextAny(user, message).catch(e => console.error('Failed to send payment link via WhatsApp:', e));
     }
 
     return res.status(200).json({ sent: true, channel });