feat(payments): payment-event dispatcher + the provider-return UX

Deliver payment_events to connected clients as an in-app wallet-refresh push: a
background dispatcher drains undispatched events and publishes a KindNotification
"payment" signal, marking each delivered; the client bumps a wallet-refresh
counter the open Wallet screen watches, re-fetching in place. A return-focus
refetch is the fallback. The Robokassa Success/Fail return now serves a
self-closing page (the payment opens in a separate window) so the customer drops
back into the live app instead of a cold start. Integration test for the event
drain/mark queue.

backend/internal/payments/service_intake.go

@@ -77,3 +77,14 @@ func (s *Service) ExpireOrders(ctx context.Context) (int, error) {
 func (s *Service) RecordPaymentEvent(ctx context.Context, accountID uuid.UUID, orderID *uuid.UUID, eventType string, payload []byte) error {
 	return s.store.insertPaymentEvent(ctx, accountID, orderID, eventType, payload, s.clock())
 }
+
+// UndispatchedEvents returns up to limit payment events awaiting delivery. The dispatcher drains
+// them and marks each delivered via MarkEventDispatched.
+func (s *Service) UndispatchedEvents(ctx context.Context, limit int) ([]PaymentEvent, error) {
+	return s.store.undispatchedEvents(ctx, limit)
+}
+
+// MarkEventDispatched stamps a payment event as delivered so it is not re-sent.
+func (s *Service) MarkEventDispatched(ctx context.Context, eventID uuid.UUID) error {
+	return s.store.markEventDispatched(ctx, eventID, s.clock())
+}