fix(youtube): recognise youtu.be / shorts / embed / live URL shapes

[MukundaKatta]

Summary

• Centralise YouTube URL → video_id extraction in a single _extract_video_id helper
• Accept youtu.be/<id> short links, /shorts/<id>, /embed/<id>, /live/<id> in addition to the canonical /watch?v=...
• Accept m.youtube.com and music.youtube.com hosts
• Have convert() reuse the same helper so it no longer silently skips the transcript path for valid non-canonical URLs

Previously YouTubeConverter.accepts() only matched https://www.youtube.com/watch?, so any other valid YouTube URL shape fell through to the generic HTML converter and lost the YouTube-specific title / description / transcript handling. convert() duplicated the ?v= parsing and had the same blind spot.

Fixes #1775.

Test plan

• New unit-test module tests/test_youtube_url_shapes.py covers all accepted shapes plus negative cases (vimeo, bare /watch with no v, channel URL, empty string, unknown host) — 14/14 pass
• Existing YouTube behaviour for www.youtube.com/watch?v=... unchanged
• Maintainer review of the host allowlist (I kept it conservative — happy to widen if there are other YouTube hosts you'd like to support)

🤖 Generated with Claude Code

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