Audio Backups:
- New audio_backups table in PostgreSQL (bytea, gzip compressed)
- POST /api/audio-backups — upload with gzip compression (level 6)
- GET /api/audio-backups — list user's backups
- GET /api/audio-backups/:id/audio — download decompressed audio
- DELETE /api/audio-backups/:id — delete backup
- Auto-cleanup every hour (24h expiry)
- Frontend saves to server first, falls back to IndexedDB
- Settings shows source badge (server/local) per backup
AI Corrections:
- Corrections list is now expandable — click to view original vs corrected
- Shows red "Original" and green "Corrected to" sections
- Click arrow to expand/collapse each correction
- Date shown on each correction
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