Mixdown ffmpeg failures stream silently corrupt files #280

Location: stems.py:_stream_ffmpeg
Severity: HIGH

Export ffmpeg runs with stderr=DEVNULL and streams stdout with HTTP 200 already committed. If ffmpeg dies mid-render, the user downloads a truncated WAV with no error anywhere — not in the UI, not in the log.
Capture stderr; on non-zero exit log the tail at WARNING with the job id and requested params. (Status can’t change mid-stream, but the log entry makes “my export is broken” reports solvable.)

From the health report. Part of #272 (Phase 1 — Evidence).