- Download a stream's subtitle tracks to a temp dir and load them into mpv as files instead of streaming from the source URL, so Subsync can use them as the alass reference (mirrors the Jellyfin subtitle preload)
- Detect format from content first, falling back to the URL extension
- A track that fails to download falls back to its URL so the episode still plays
- Remove the temp dir when the next episode loads or the runtime disposes
