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/utils/pricing.js

@@ -161,18 +161,18 @@ async function refreshPricingForRegistration(registrationId) {
   });
   if (!registration) return { changed: false };
 
-  let anyChanged = false;
-
-  for (const ro of registration.registrationOptions) {
+  // Each registrationOption is independent, so resolve/update them concurrently
+  // instead of one at a time — this loop sits directly in the payment-capture path.
+  const results = await Promise.all(registration.registrationOptions.map(async (ro) => {
     // Only refresh options that were priced via a tier
-    if (!ro.appliedTierId) continue;
+    if (!ro.appliedTierId) return false;
 
     // Find the currently applied tier
     const currentTier = (ro.eventOption.earlyBirdTiers || []).find(t => t.id === ro.appliedTierId);
 
     if (currentTier && new Date() < new Date(currentTier.deadline)) {
       // The tier's deadline is still in the future — honor the locked price.
-      continue;
+      return false;
     }
 
     // Deadline has passed (or tier record missing) — resolve the next applicable tier
@@ -191,11 +191,12 @@ async function refreshPricingForRegistration(registrationId) {
           appliedTierId: resolved.tierId
         }
       });
-      anyChanged = true;
+      return true;
     }
-  }
+    return false;
+  }));
 
-  return { changed: anyChanged };
+  return { changed: results.some(Boolean) };
 }
 
 /**