_pitch_preserving_stretch ran a blocking subprocess.run() inside the
`_stream` async generator (on the event loop). Each ffmpeg atempo call is
~50-100 ms, so on a multi-segment time_stretch dub job it froze health
checks, status SSE, and every other concurrent request for seconds.
Convert to asyncio.create_subprocess_exec + await communicate() (same
pattern as run_proc_streaming_stderr); await the call site in _stream.
Drop the now-unused `import subprocess`.
Tests: async coroutine + target-length + no-op cases (real ffmpeg).
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
