- Add sender_type field to TypingEvent (user/ai)
- Change Redis TTL from 10s to 60s for AI typing persistence
- Broadcast typing.start/stop with sender_type=ai when AI stream starts/ends
- Replay active AI typing events from Redis on new WS subscribe
- Fix ai.stream_chunk WS payload missing display_name and chunk_type
- Add initial thinking chunk on AI stream start for immediate indicator