Cloudreve: Non-admin remote download users can SSRF loopback/internal services and read imported responses

Cloudreve is a self-hosted file management and sharing system. Prior to 4.16.1, Cloudreve's remote download workflow accepts user-supplied URLs at POST /api/v4/workflow/download and passes them to the configured downloader without blocking loopback, localhost, IPv6 localhost, or redirect-to-loopback targets, allowing a non-admin user with remote download permission to fetch internal-only URLs and read the response after it is imported into the user's own files. This issue is fixed in version 4.16.1.