fix(haproxy): Validate backend names, reject IP:port misconfiguration

Add validation in _add_vhost_acl() to detect when vhost backend
is set to IP:port format instead of a proper backend name.

This prevents haproxy config generation errors like:
  [ALERT] unable to find required use_backend: '127.0.0.1:8081'

When detected, logs a warning and skips the vhost instead of
generating invalid config that crashes HAProxy.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package/secubox/secubox-app-haproxy/files/usr/sbin/haproxyctl

@@ -553,6 +553,10 @@ _add_vhost_acl() {
 
 	config_get domain "$section" domain
 	config_get backend "$section" backend
+	# Validate backend is not IP:port (common misconfiguration)
+	case "$backend" in
+		*:*) log_warn "Vhost $section has IP:port backend , should be backend name"; return ;;
+	esac
 	config_get ssl "$section" ssl "0"
 
 	[ -n "$domain" ] || return