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/prisma/schema.prisma

@@ -201,6 +201,7 @@ model Registration {
   @@index([eventId])
   @@index([userId, status])
   @@index([eventId, status])
+  @@index([createdAt, status])
 }
 
 model RegistrationOption {
@@ -249,7 +250,7 @@ model Payment {
   @@index([registrationId])
   @@index([eventId])
   @@index([createdAt])
-  @@index([originalPaymentId])
+  @@index([originalPaymentId, isDonation])
 }
 
 model Ticket {
@@ -271,6 +272,7 @@ model Ticket {
   @@index([eventId])
   @@index([userId])
   @@index([registrationOptionId])
+  @@index([createdAt])
 }
 
 model TicketUsage {