refactor machine name generation using secrets package

internal/machine/cluster/machine.go

@@ -1,9 +1,7 @@
 package cluster
 
 import (
-	"crypto/rand"
 	"fmt"
-	"math/big"
 	"uncloud/internal/secret"
 )
 
@@ -14,14 +12,9 @@ func NewMachineID() (string, error) {
 
 // NewRandomMachineName generates a random machine name in the format "machine-xxxx".
 func NewRandomMachineName() (string, error) {
-	const charset = "abcdefghijklmnopqrstuvwxyz0123456789"
-	suffix := make([]byte, 4)
-	for i := range suffix {
-		randIdx, err := rand.Int(rand.Reader, big.NewInt(int64(len(charset))))
-		if err != nil {
-			return "", fmt.Errorf("get random number: %w", err)
-		}
-		suffix[i] = charset[randIdx.Int64()]
+	suffix, err := secret.RandomAlphaNumeric(4)
+	if err != nil {
+		return "", fmt.Errorf("generate random suffix: %w", err)
 	}
-	return "machine-" + string(suffix), nil
+	return "machine-" + suffix, nil
 }