- Add POST /transcribe/sessions/{id}/export endpoint
- Generate SRT (SubRip subtitle format) with timestamps
- Generate TXT (plain text with [HH:MM:SS,mmm] timestamps)
- Generate JSON (structured data: session, segments, summaries, canvas events)
- Return as FileResponse download with Content-Disposition headers
- Filenames include sanitized session title + date
- No API keys stored or logged during export