fix: scope the connect-time loopback allowance to the configured proxy

The connect-time guard permitted every loopback address so that a locally
configured proxy (proxy: http://127.0.0.1:9050) stayed reachable. But the
connect guard is the only check that media URLs derived from remote metadata
ever face — validate_url sees just the submitted URL — so that blanket
allowance let a remote manifest steer the download subprocess at services
bound to the server's loopback interface, with the response written to the
download directory and served back by the UI.

Permit loopback only at the host:port of a proxy the operator configured,
taken from yt-dlp's proxy option and the *_proxy environment variables.
Nothing is lost: when a proxy is in use yt-dlp hands it the media URL instead
of resolving that URL locally, so the two cases never overlap. Every other
loopback destination now falls under the same is_global policy as the rest.

Reported by m3rl1nu5 (https://github.com/hai135) in GHSA-73g4-qhhq-c32c.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/ytdl.py

@@ -657,10 +657,12 @@ class Download:
             except OSError:
                 pass
         # Re-validate every outbound connection at fetch time. validate_url only
-        # saw the submitted URL string; this catches redirects and DNS rebinding
-        # to internal hosts (cloud metadata, RFC1918) that it cannot. Skipped when
-        # ALLOW_PRIVATE_ADDRESSES trusts the environment (e.g. Fake-IP proxies).
-        install_socket_guard(self.allow_private)
+        # saw the submitted URL string; this catches redirects, DNS rebinding and
+        # attacker-controlled media URLs pulled from a remote manifest, none of
+        # which it can see. The configured proxy is passed so that a proxy on
+        # loopback stays reachable at its own address without opening up the rest
+        # of loopback. Skipped when ALLOW_PRIVATE_ADDRESSES trusts the environment.
+        install_socket_guard(self.allow_private, proxy_urls=(self.ytdl_opts.get('proxy'),))
         log.info(f"Starting download for: {self.info.title} ({self.info.url})")
         try:
             debug_logging = logging.getLogger().isEnabledFor(logging.DEBUG)