Add subtitle and caption extraction for media downloads

[thsnkhn]

Problem

Harbor can download public media, but users cannot request subtitles or captions from supported sites such as YouTube.

Goal

Allow users to download available subtitle tracks with a media download.

Acceptance criteria

• Media preview lists available subtitle languages when yt-dlp provides them.
• Users can choose one or more subtitle languages.
• The UI distinguishes creator-provided subtitles from automatic captions.
• The first version can save subtitles as sidecar files.
• Users can choose a common format such as VTT or SRT when conversion is available.
• A missing subtitle track does not fail the media download.
• Completed subtitle files appear with the downloaded media in the destination folder.

Source

Requested in the Harbor 1.7.0 Reddit announcement.