Add Extract Embedded Subtitles plugin

[loco306]

Extract Embedded Subtitles

A new external plugin that extracts embedded text subtitle tracks from video files into external .srt sidecar files, named so Stash's existing caption matcher attaches them automatically.

Addresses the long-standing request in stashapp/stash#3875. video.js can't render embedded tracks directly, so the practical path is generating sidecar .srt files - this automates that.

What it does

• ffprobe enumerates each scene file's subtitle streams.
• Each text track (SubRip / ASS / SSA / mov_text / WebVTT / .) is extracted via ffmpeg -map 0:<idx> -c:s srt to <video>.<lang>.srt, matching GetCaptionPath in pkg/file/video/caption.go (ISO-639 language segment, or no segment when the language is unknown).
• Touched folders are rescanned (metadata_scan) so captions appear without a full library rescan.
• Provides the Extract Embedded Subtitles task (whole library), an optional Scene.Create.Post hook (off by default), and Overwrite / Extract On Scan settings.

Limitations (documented in the README)

• Image-based subtitles (PGS / VobSub / DVB) are skipped - they're bitmaps and need OCR.
• ASS/SSA styling is lost when converting to SRT (predictable; the source file is never modified).
• One caption per language, since Stash stores one caption per language/type; the default track wins ties.

Testing

• Manifest validated against validator/plugin.schema.json (Draft-07) - passes.
• Extraction verified end-to-end against real ffmpeg/ffprobe on a multi-track MKV: English (SubRip), Spanish (muxed as ASS to exercise ASS?SRT), and an untagged track ? produced name.eng.srt, name.spa.srt, name.srt with text and timing intact; re-run is idempotent (skips existing).
• Filename / ISO-639 logic unit-tested against the rules in caption.go.
• Not yet exercised inside a running Stash server; caption attachment relies on Stash's existing external-subtitle support plus metadata_scan.

Disclosure

Per the repo's LLM-assisted contributions policy: this was LLM-assisted, human-reviewed, and tested (details above). The code is original - it shells out to ffmpeg/ffprobe and uses stashapp-tools like other plugins here - and I take responsibility for it.

Relates to: stashapp/stash#3875