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/util/util.go

@@ -0,0 +1,18 @@
+package util
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"fmt"
+	"time"
+)
+
+// RandomHex генерирует случайную hex-строку из n байт.
+// При ошибке crypto/rand использует time-based fallback.
+func RandomHex(n int) string {
+	b := make([]byte, n)
+	if _, err := rand.Read(b); err != nil {
+		return fmt.Sprintf("%x", time.Now().UnixNano())[:n*2]
+	}
+	return hex.EncodeToString(b)[:n*2]
+}