Regenerate

Regenerate conversation from a specific message. Starts background task and returns immediately.

Request body: {"message_id": "uuid", "message": "optional new content"}

Events are broadcast to SSE room:

• message_updated: {updated message if content changed}
• delete_message: {deleted message id}
• text, tool_started, tool_delta: streaming events
• message: {new conversation messages}
• done: {}