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/migrations/20260807215056_add_perf_indexes/migration.sql

@@ -0,0 +1,11 @@
+-- DropIndex
+DROP INDEX "Payment_originalPaymentId_idx";
+
+-- CreateIndex
+CREATE INDEX "Payment_originalPaymentId_isDonation_idx" ON "Payment"("originalPaymentId", "isDonation");
+
+-- CreateIndex
+CREATE INDEX "Registration_createdAt_status_idx" ON "Registration"("createdAt", "status");
+
+-- CreateIndex
+CREATE INDEX "Ticket_createdAt_idx" ON "Ticket"("createdAt");