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/inttest/payments_intake_test.go

@@ -77,6 +77,58 @@ func TestPaymentsOrderFundCreditsOnce(t *testing.T) {
 	}
 }
 
+// TestPaymentsVKOrderFundCredits exercises the VK Votes rail over Postgres: a VK order prices the
+// pack in votes; the get_item lookup returns its title and vote price; a chargeable
+// order_status_change credits the vk segment exactly once (idempotent on VK's own order id).
+func TestPaymentsVKOrderFundCredits(t *testing.T) {
+	ctx := context.Background()
+	svc := newPaymentsService()
+	acc := uuid.New()
+	prod := seedPackProduct(t, 200, methodPrice{method: "vk", currency: "VOTE", amount: 30})
+
+	res, err := svc.CreateOrder(ctx, acc, payments.NewContext("vk", "web"), []payments.Source{payments.SourceVK}, prod, "vk")
+	if err != nil {
+		t.Fatalf("create vk order: %v", err)
+	}
+	if res.Amount.Currency() != payments.CurrencyVote || res.Amount.Minor() != 30 {
+		t.Fatalf("vk order amount = %s, want 30 VOTE", res.Amount)
+	}
+
+	// get_item phase: title + vote price.
+	title, amount, err := svc.OrderItem(ctx, res.OrderID)
+	if err != nil {
+		t.Fatalf("order item: %v", err)
+	}
+	if title == "" || amount.Minor() != 30 || amount.Currency() != payments.CurrencyVote {
+		t.Errorf("order item = %q / %s, want a title + 30 VOTE", title, amount)
+	}
+
+	// order_status_change phase: VK's own order id is the idempotency key.
+	paid, _ := payments.MoneyFromMinor(30, payments.CurrencyVote)
+	out, err := svc.Fund(ctx, res.OrderID, "vk", "vk-order-777", paid)
+	if err != nil {
+		t.Fatalf("vk fund: %v", err)
+	}
+	if out.AlreadyCredited || out.Chips != 200 || out.Source != payments.SourceVK {
+		t.Fatalf("vk fund outcome = %+v, want 200 chips credited to vk", out)
+	}
+	if got := readBalance(t, acc, "vk"); got != 200 {
+		t.Errorf("vk balance after fund = %d, want 200", got)
+	}
+
+	// A duplicate VK callback (same VK order id) credits nothing more.
+	out2, err := svc.Fund(ctx, res.OrderID, "vk", "vk-order-777", paid)
+	if err != nil {
+		t.Fatalf("duplicate vk fund: %v", err)
+	}
+	if !out2.AlreadyCredited {
+		t.Error("duplicate VK callback not flagged AlreadyCredited")
+	}
+	if got := readBalance(t, acc, "vk"); got != 200 {
+		t.Errorf("vk balance after duplicate = %d, want 200 (credited once)", got)
+	}
+}
+
 // TestPaymentsFundAmountMismatch verifies a callback whose paid amount does not match the order is
 // refused and credits nothing (§9: verify the amount after matching by order id).
 func TestPaymentsFundAmountMismatch(t *testing.T) {