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.

gateway/cmd/gateway/main.go

@@ -208,6 +208,7 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 		Tracker:      tracker,
 		Banlist:      banlist,
 		Honeytoken:   cfg.Abuse.Honeytoken,
+		VKAppSecret:  cfg.VKAppSecret,
 		Hub:          hub,
 		RateLimit:    cfg.RateLimit,
 		Heartbeat:    cfg.PushHeartbeatInterval,