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>

backend/requirements.txt

@@ -4,6 +4,7 @@ python-dotenv>=1.0.0
 mysql-connector-python>=9.0.0
 pydantic>=2.10.0
 httpx>=0.27.0
+python-multipart>=0.0.9
 python-jose[cryptography]>=3.3.0
 openpyxl>=3.1.2
 pytest>=8.0.0