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/app/config.py

@@ -16,6 +16,7 @@ class Settings:
     )
     DIFY_API_URL: str = os.getenv("DIFY_API_URL", "https://dify.theaken.com/v1")
     DIFY_API_KEY: str = os.getenv("DIFY_API_KEY", "")
+    DIFY_STT_API_KEY: str = os.getenv("DIFY_STT_API_KEY", "")
 
     ADMIN_EMAIL: str = os.getenv("ADMIN_EMAIL", "ymirliu@panjit.com.tw")
     JWT_SECRET: str = os.getenv("JWT_SECRET", "meeting-assistant-secret")