Add Chat to Remote Support Sessions

## Overview Add real-time text chat capability to remote support sessions, enabling operators and guests to exchange messages alongside the video call. ## Architecture Decisions 1. **Chat Model**: Session-scoped chat - each `SupportSession` has its own message stream via a new `SupportSessionMessage` model 2. **Persistence**: Messages are persisted for audit and review after session ends 3. **Guest Transport**: WebSocket fallback endpoint for unauthenticated guest subscriptions (guests have no NATS device) 4. **UI Placement**: Collapsible panel alongside video in both operator and guest interfaces ## Key Challenges - Guest is unauthenticated (no `User` record) - messages need `participant_type` field - Guest cannot use NATS (no device context) - requires dedicated WebSocket/SSE endpoint - Must work with existing Daily.co video infrastructure - Chat should remain accessible after session ends (for operators reviewing history) ## Success Criteria - [ ] Operators can send/receive messages during active sessions - [ ] Guests can send/receive messages without authentication - [ ] Messages persist and are viewable after session ends - [ ] Real-time delivery with minimal latency - [ ] Consistent collapsible panel UI on both sides