feat(payments): VK Votes payment rail

Wire the VK Mini Apps ("голоса") rail end to end, reusing the intake engine. The
wallet.order endpoint branches by rail: a VK context opens a pending order
(provider vk) and returns its id, which the client passes to VKWebAppShowOrderBox.
VK's two-phase payment callback is verified at the gateway with the app protected
key (GATEWAY_VK_APP_SECRET) and proxied to a backend intake handler: get_item
returns the ordered pack's title and vote price; a chargeable order_status_change
credits the vk segment exactly once (the same Fund, idempotent on VK's own order
id) and records a succeeded event, so the dispatcher push refreshes the wallet.
Integration test for the VK order->credit path.

backend/internal/payments/service_intake.go

@@ -52,6 +52,25 @@ func (s *Service) CreateOrder(ctx context.Context, accountID uuid.UUID, cxt Cont
 	return OrderResult{OrderID: orderID, Amount: pack.price, Title: pack.title}, nil
 }
 
+// OrderItem returns a pending order's human title and the amount it charges, in the order's own
+// currency — the details a provider's item-lookup phase needs (VK's get_item). It reads the order
+// and the pack title, honouring the pack even if it was later deactivated (mirrors Fund).
+func (s *Service) OrderItem(ctx context.Context, orderID uuid.UUID) (title string, amount Money, err error) {
+	ord, err := s.store.orderByID(ctx, orderID)
+	if err != nil {
+		return "", Money{}, err
+	}
+	_, title, err = s.store.packForCredit(ctx, ord.productID)
+	if err != nil {
+		return "", Money{}, err
+	}
+	amount, err = MoneyFromMinor(ord.expectedAmount, Currency(ord.currency))
+	if err != nil {
+		return "", Money{}, err
+	}
+	return title, amount, nil
+}
+
 // Fund credits a paid order into its funded segment exactly once, from a verified provider callback
 // — the single writer for every rail. It matches the order, verifies the paid amount, appends the
 // fund ledger row (idempotent on (provider, provider_payment_id)), credits the balance and marks