User-submitted URLs were passed straight to yt-dlp's generic extractor,
letting the server fetch internal endpoints (cloud metadata, loopback,
RFC1918 hosts). Add a url_guard.validate_url check at every URL ingress
(add, subscribe, and nested playlist recursion) that rejects non-http(s)
schemes and hosts resolving to non-global addresses, while leaving bare
video IDs and search prefixes untouched.
