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/cmd/backend/main.go

@@ -320,6 +320,9 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 	// Sweep expired pending payment orders on a cadence (cosmetic hygiene; a late valid callback
 	// still credits). Runs until ctx is cancelled.
 	go runOrderReaper(ctx, paymentsSvc, logger)
+	// Deliver pending payment_events to connected clients as an in-app wallet-refresh push (the
+	// credit already landed in the ledger; a return-focus poll is the client-side fallback).
+	go runPaymentDispatcher(ctx, paymentsSvc, hub, logger)
 
 	errc := make(chan error, 2)
 	go func() { errc <- pushSrv.Run(ctx) }()
@@ -350,6 +353,32 @@ func runOrderReaper(ctx context.Context, p *payments.Service, log *zap.Logger) {
 	}
 }
 
+// runPaymentDispatcher delivers pending payment_events to connected clients as a wallet-refresh
+// signal (KindNotification / "payment"), marking each delivered, until ctx is cancelled. The credit
+// already committed to the ledger; this is only the in-app push so an open wallet updates in place.
+func runPaymentDispatcher(ctx context.Context, p *payments.Service, pub notify.Publisher, log *zap.Logger) {
+	t := time.NewTicker(3 * time.Second)
+	defer t.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-t.C:
+			evs, err := p.UndispatchedEvents(ctx, 50)
+			if err != nil {
+				log.Warn("payment dispatcher: read failed", zap.Error(err))
+				continue
+			}
+			for _, e := range evs {
+				pub.Publish(notify.Notification(e.AccountID, notify.NotifyPayment))
+				if err := p.MarkEventDispatched(ctx, e.EventID); err != nil {
+					log.Warn("payment dispatcher: mark failed", zap.String("event", e.EventID.String()), zap.Error(err))
+				}
+			}
+		}
+	}
+}
+
 // newMailer builds the confirm-code mailer: an SMTP relay when a host is
 // configured, otherwise the development log mailer (the code is logged, not sent).
 func newMailer(cfg account.SMTPConfig, logger *zap.Logger) account.Mailer {