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/inttest/payments_intake_test.go

@@ -101,6 +101,45 @@ func TestPaymentsFundAmountMismatch(t *testing.T) {
 	}
 }
 
+// TestPaymentsEventDispatchDrain verifies the payment_events dispatcher queue: a recorded event is
+// returned as undispatched until marked, then drops out (so the dispatcher delivers it once).
+func TestPaymentsEventDispatchDrain(t *testing.T) {
+	ctx := context.Background()
+	svc := newPaymentsService()
+	acc := uuid.New()
+	if err := svc.RecordPaymentEvent(ctx, acc, nil, "succeeded", []byte(`{"chips":10,"source":"direct"}`)); err != nil {
+		t.Fatalf("record event: %v", err)
+	}
+
+	// testDB is shared, so filter the queue to our account.
+	find := func() *payments.PaymentEvent {
+		evs, err := svc.UndispatchedEvents(ctx, 100)
+		if err != nil {
+			t.Fatalf("undispatched: %v", err)
+		}
+		for i := range evs {
+			if evs[i].AccountID == acc {
+				return &evs[i]
+			}
+		}
+		return nil
+	}
+
+	mine := find()
+	if mine == nil {
+		t.Fatal("recorded event not in the undispatched queue")
+	}
+	if mine.Type != "succeeded" {
+		t.Errorf("event type = %s, want succeeded", mine.Type)
+	}
+	if err := svc.MarkEventDispatched(ctx, mine.EventID); err != nil {
+		t.Fatalf("mark dispatched: %v", err)
+	}
+	if find() != nil {
+		t.Error("event still undispatched after MarkEventDispatched")
+	}
+}
+
 // TestPaymentsExpiredOrderStillCredits verifies an expired pending order is still honoured by a
 // later valid callback (§9/D23: expiry is cosmetic, the money is real).
 func TestPaymentsExpiredOrderStillCredits(t *testing.T) {