v0.20.0: Kept summaries + Listen

Every summary is now kept for your account, so asking for the same document (or change) again, on any device or after a reload, costs nothing and makes no new AI call. New Listen button: hear a summary read aloud (ElevenLabs). The first listen is paid from your balance and replays are free. API: POST /api/v1/summaries//speech returns MP3.