Expert Chat Persistence across sessions

**Parent Deliverable:** FlowFuse/product#47 — Expert as Agentic Co-pilot

## Context

From the March 11/13 refinement whiteboards. The Expert needs to persist chat history across sessions so users can resume multi-step tasks. Currently, chat context is lost when the session ends or the page is refreshed.

Related considerations from the MQTT architecture discussion (whiteboard 3):
- Shared sessions need rehydration
- New sessions from old session parts need rehydration as well
- MQTT migration (#6897) enables better session management via topic-based routing

## Scope

- Persist Expert chat history so it survives page refresh and session boundaries
- Enable session rehydration: user can return to a previous conversation context
- Support multi-step agentic tasks that may span multiple interactions
- Consider storage approach: server-side vs client-side persistence

## Dependencies

- MQTT migration (#6897) — topic-based session management may inform persistence design
- Forge BE — may need API endpoints for chat history storage/retrieval

## Acceptance Criteria

- [ ] Chat history persists across page refreshes
- [ ] User can resume a previous Expert conversation with full context
- [ ] Multi-step tasks (e.g., scaffolding a flow across multiple prompts) maintain context
- [ ] Old sessions can be referenced when starting new ones
- [ ] Storage approach handles reasonable chat history sizes without performance degradation