Implement prodl-core shared package

[ClausValcaTD]

Builds the prodl-core package containing shared Pydantic v2 models, ProDLDownloader (yt-dlp wrapper using asyncio threadpools), async thumbnail fetcher, subtitle lister, and SponsorBlock segment fetcher. All changes are restricted to packages/prodl-core/.

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PR created automatically by Jules for task 11552008329511297475 started by @ClausValcaTD

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