feat: Add Dify audio transcription with VAD chunking and SSE progress

- Add audio file upload transcription via Dify STT API
- Implement VAD-based audio segmentation in sidecar (3-min chunks)
- Add SSE endpoint for real-time transcription progress updates
- Fix chunk size enforcement for reliable uploads
- Add retry logic with exponential backoff for API calls
- Support Python 3.13+ with audioop-lts package
- Update frontend with Chinese progress messages and chunk display
- Improve start.sh health check with retry loop

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

sidecar/requirements.txt

@@ -3,3 +3,5 @@ faster-whisper>=1.0.0
 opencc-python-reimplemented>=0.1.7
 numpy>=1.26.0
 onnxruntime>=1.16.0
+pydub>=0.25.0
+audioop-lts>=0.2.1  # Required for Python 3.13+ (audioop removed from stdlib)