AVideo CVE-2026-43881 incomplete fix - `objects/mention.json.php:17` is an unauthenticated user enumeration sibling that survives `d9cdc7024`

CVE-2026-43881 fix `d9cdc7024` patched `users.json.php` only. The same anti-pattern survives at master HEAD in: ``` objects/mention.json.php:17 $ignoreAdmin = true; objects/mention.json.php:18 $users = User::getAllUsers($ignoreAdmin, ['name', 'email', 'user', 'channelName'], 'a'); ``` No `User::loginCheck()`, no admin gate. Only entry guard: `preg_match('/^@/', $_REQUEST['term'])` and hard-coded `rowCount=10`.