Add Extract Embedded Subtitles plugin (#734)

* Add Extract Embedded Subtitles plugin

Extracts embedded text subtitle tracks from video files into external
.srt sidecar files that Stash recognises as captions (stashapp/stash#3875).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Refactor README for clarity

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Co-authored-by: Caleb Dean <realcalebdean@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: DogmaDragon <103123951+DogmaDragon@users.noreply.github.com>