# Change: Add Dify Audio Transcription for Uploaded Files

## Why
Users need to transcribe pre-recorded audio files (e.g., meeting recordings from external sources). Currently, transcription only works with real-time recording via the local sidecar. Adding Dify-based transcription for uploaded files provides flexibility while keeping real-time transcription local for low latency.

## What Changes
- Add audio file upload UI in Electron client (meeting detail page)
- Add `segment_audio` command to sidecar for VAD-based audio chunking
- Add backend API endpoint to receive audio files, chunk via sidecar, and forward to Dify STT service
- Each chunk (~5 minutes max) sent to Dify separately, results concatenated
- Transcription result replaces the transcript field (same as real-time transcription)
- Support common audio formats: MP3, WAV, M4A, WebM, OGG

## Impact
- Affected specs: `transcription`
- Affected code:
  - `sidecar/transcriber.py` - Add `segment_audio` action for VAD chunking
  - `client/src/pages/meeting-detail.html` - Add upload button and progress UI
  - `backend/app/routers/ai.py` - Add `/api/ai/transcribe-audio` endpoint
  - `backend/app/config.py` - Add Dify STT API key configuration

## Technical Notes
- Dify STT API Key: `app-xQeSipaQecs0cuKeLvYDaRsu`
- Real-time transcription continues to use local sidecar (no change)
- File upload transcription uses Dify cloud service with VAD chunking
- VAD chunking ensures each chunk < 25MB (Dify API limit)
- Max file size: 500MB (chunked processing handles large files)
- Both methods output to the same transcript_blob field