refactor: common util package, ESL/AMI/security fixes, Prometheus metrics

Backend stability and security improvements:

* internal/util/ — common RandomHex helper, removed 3 duplicates
* ESL: deduplicated readMessage (locked/unlocked), net.JoinHostPort for IPv6
* AMI: synchronous reconnect() in readEventsLoop, net.JoinHostPort for IPv6
* Auth: /api/auth/refresh accepts Authorization header only (no ?token=)
* decodeJSON: http.MaxBytesReader(1<<20) body limit
* Trunks: gatewayParams() uses configured ESL.GatewayPrefix
* Config: jwt_secret_env env-var fallback
* FSCollector: time.After → time.NewTimer with defer Stop
* Monitoring: Prometheus counters (route_requests, nodes_total/healthy, uptime)
* go fmt pass across all internal/ packages

internal/engine/engine.go

@@ -190,11 +190,11 @@ func (e *Engine) IncrementRouteRequests() {
 
 // HealthStats возвращает статистику для health endpoint.
 type HealthStats struct {
-	TotalNodes     int `json:"total_nodes"`
-	HealthyNodes   int `json:"healthy_nodes"`
-	RouteRequests  int64 `json:"route_requests_total"`
-	UptimeSeconds  int64 `json:"uptime_seconds"`
-	LastMetricTS   int64 `json:"last_metric_ts"`
+	TotalNodes    int   `json:"total_nodes"`
+	HealthyNodes  int   `json:"healthy_nodes"`
+	RouteRequests int64 `json:"route_requests_total"`
+	UptimeSeconds int64 `json:"uptime_seconds"`
+	LastMetricTS  int64 `json:"last_metric_ts"`
 }
 
 // GetHealthStats возвращает текущую HealthStats (потокобезопасно).
@@ -209,11 +209,11 @@ func (e *Engine) GetHealthStats() HealthStats {
 		}
 	}
 	return HealthStats{
-		TotalNodes:     len(e.nodes),
-		HealthyNodes:   healthy,
-		RouteRequests:  e.routeRequests.Load(),
-		UptimeSeconds:  int64(time.Since(e.startTime).Seconds()),
-		LastMetricTS:   e.lastMetricTime.Load(),
+		TotalNodes:    len(e.nodes),
+		HealthyNodes:  healthy,
+		RouteRequests: e.routeRequests.Load(),
+		UptimeSeconds: int64(time.Since(e.startTime).Seconds()),
+		LastMetricTS:  e.lastMetricTime.Load(),
 	}
 }
 

internal/engine/engine_test.go

@@ -12,7 +12,7 @@ func defaultCfg() *config.Config {
 	return &config.Config{
 		StaleThresholdSec: 20,
 		Scoring: config.ScoringConfig{
-			IdleCPUMin:           5.0,
+			IdleCPUMin:            5.0,
 			CallFailureRateLethal: 15.0,
 			Weights: config.ScoringWeights{
 				CallScore: 0.40,