fix(security): restrict youtube-audio to YouTube hostnames

[carochacs]

The route handed a caller-supplied URL straight to yt_dlp, which falls
back to its generic extractor for anything it doesn't recognize — that
extractor can fetch essentially any web page server-side, an SSRF shape
with no scheme/host restriction. The UI only ever offers "paste a
YouTube URL" (screen.html), so add _is_youtube_url() and reject any
request whose scheme isn't http/https or whose host isn't a real
youtube.com/youtu.be hostname with 400, before yt_dlp ever sees the URL.

Closes #14.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com

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[pullfrog]

✅ No new issues found.

Reviewed changes

This PR fixes an SSRF vulnerability in the youtube-audio endpoint (issue #14). The route previously passed caller-supplied URLs to yt_dlp without validation, allowing yt_dlp's generic extractor to fetch arbitrary pages server-side.

• SSRF guard (_is_youtube_url) — Adds a whitelist-based hostname validator at routes.py:1047-1066 using urlparse. Checks scheme (http/https) and hostname against a frozenset of known YouTube hostnames. Pure string validation, no network access.
• Route guard — Applies _is_youtube_url as an early-return in the youtube_audio handler at routes.py:6757-6758, returning 400 before yt_dlp ever sees the URL.
• Comprehensive tests — New tests/test_youtube_audio_url.py (74 lines) covers valid YouTube URLs, non-YouTube hosts (including 169.254.169.254 cloud metadata), lookalike domains, userinfo confusion (youtube.com@evil.example), non-HTTP schemes, and malformed input.
• Version bump and changelog — plugin.json bumped to 1.8.3, security entry added to CHANGELOG.md.

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