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/models/types.go

@@ -15,6 +15,7 @@ type NodeMetric struct {
 	IdleCPU          float64 `json:"idle_cpu"`           // 0..100 %
 	LoadAvg          float64 `json:"load_avg"`           // system load average (1m)
 	CallFailureRate  float64 `json:"call_failure_rate"`  // 0.0..100.0 %
+	SIPGateway       string  `json:"sip_gateway,omitempty"` // SIP-адрес ноды для авто-создания транка
 }
 
 // --- In-memory состояние ноды в engine ---