- POST /api/audio/fetch accepts {url: string}
- Spawns yt-dlp subprocess via sys.executable -m yt_dlp
- Streams SSE progress events with PROGRESS_RE line-by-line parsing
- Uses --newline flag for reliable progress output
- Uses yt_%(id)s.%(ext)s template for predictable filenames
- Runs beat analysis on completion and caches in app.state.beats
- Returns done event with path and filename on success
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