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/.env.example

@@ -9,6 +9,7 @@ DB_NAME=db_A060
 AUTH_API_URL=https://pj-auth-api.vercel.app/api/auth/login
 DIFY_API_URL=https://dify.theaken.com/v1
 DIFY_API_KEY=app-xxxxxxxxxxx
+DIFY_STT_API_KEY=app-xxxxxxxxxxx
 
 # Application Settings
 ADMIN_EMAIL=ymirliu@panjit.com.tw