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

cmd/emulator/main.go

@@ -49,7 +49,7 @@ type valueSpec struct {
 	isRand bool
 }
 
-func fixed(v float64) valueSpec   { return valueSpec{fixed: v} }
+func fixed(v float64) valueSpec { return valueSpec{fixed: v} }
 func rrange(min, max float64) valueSpec {
 	return valueSpec{min: min, max: max, isRand: true}
 }
@@ -71,7 +71,7 @@ type nodeSpec struct {
 	idleCPU    valueSpec
 	loadAvg    valueSpec
 	failRate   valueSpec
-	sipGateway string   // SIP-адрес для авто-создания транка
+	sipGateway string // SIP-адрес для авто-создания транка
 }
 
 func (ns *nodeSpec) generate() nodeMetric {