feat: auto-trunk creation, SIP gateway in metrics, gateway WS broadcast

- SIPGateway field in NodeMetric model
- HasNode() engine method
- OnNewNode callback in NATS subscriber
- Auto-create balance trunk from new node metric
- BroadcastGatewayEvent on WS hub
- Gateway status indicators in trunks UI (green/red/gray)
- SPA fallback for frontend routing

internal/engine/engine.go

@@ -144,6 +144,14 @@ func (e *Engine) GetNodeHistory(nodeID string) []NodeSnapshot {
 	return ring.snapshot()
 }
 
+// HasNode возвращает true, если нода уже зарегистрирована в engine.
+func (e *Engine) HasNode(nodeID string) bool {
+	e.mu.RLock()
+	defer e.mu.RUnlock()
+	_, exists := e.nodes[nodeID]
+	return exists
+}
+
 // ToggleNode включает/выключает ноду из распределения.
 func (e *Engine) ToggleNode(nodeID string, disabled bool, reason string) error {
 	e.mu.Lock()