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/nats/subscriber.go

@@ -9,9 +9,9 @@ import (
 
 	natsgo "github.com/nats-io/nats.go"
 
-	"github.com/pulse-lets-go/internal/models"
 	"github.com/pulse-lets-go/internal/engine"
 	filelog "github.com/pulse-lets-go/internal/log"
+	"github.com/pulse-lets-go/internal/models"
 )
 
 const subject = "pulse.metrics.>"