fix(youtube): auto-detect caption language instead of hardcoding English

[TTigger]

Problem

The yt-dlp provider requested --sub-langs en.* for every video. For a non-English channel this meant transcripts were either dropped, or — worse — distilled from YouTube's English auto-translation, losing the speaker's actual voice. (Found while distilling a Mandarin lecture channel: the mask came out in translated English instead of the speaker's Chinese.)

Fix

Auto-detect the caption language instead of hardcoding it:

• Probe each video's available tracks with yt-dlp -J (subtitles, automatic_captions, language).
• New pure, unit-tested pickSubtitleLang() chooses, in order:
  1. an explicit override (MASK_YT_SUBLANGS),
  2. the video's original language (yt-dlp language), human-authored track first,
  3. any human-authored (manual) track,
  4. the original auto-caption track.
• Machine-translated auto tracks (keyed <target>-<source>, e.g. en-zh-TW) are avoided so a Chinese lecture isn't distilled from its English auto-translation. When language is absent, the original is inferred as the source shared across the <target>-<source> translation keys.
• MASK_YT_SUBLANGS remains as an escape hatch: an explicit yt-dlp --sub-langs expression (comma lists / zh.* wildcards) passed through verbatim.

Design notes

• Selection logic is extracted as an exported pure function and tested offline, matching the existing parseVtt / isVideoUrl pattern — the live yt-dlp path stays thin and the injectable-provider contract is unchanged (existing fakes still work).
• Cost: one extra yt-dlp -J metadata call per video (correctness over speed).

Tests

• 5 new pickSubtitleLang cases (original-with-manual, original-auto-over-translations, infer-without-language, English-original / lone-auto, override + null). bun test test/youtube.test.ts → 10 pass.
• bun run typecheck and bun run build green. Full suite unchanged except the new passing tests (any local failures are pre-existing Windows-only path issues; CI runs on Linux).

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