Send a message to the assistant (text/event-stream)

Body: {message, context: {module: showroom|backoffice|settings, screen?, route?, selection?: {type, id}, audience?: staff|client, client?: {id, name}, design?: {capsule_id, values, values_resolved, selected_part, design_id, draft_id, mode: studio|fit}}, title?, kickoff?}. With id "new" a conversation is created (titled with title or the message) and announced in the first event; opened from the Showroom it is ephemeral (ephemeral: true): not listed, and the app deletes it when the counter session ends. context.design makes it a Design Studio turn: the assistant gets the piece, the Studio toolset and the UI tools update_record / switch_capsule, streamed as ui_tool events for the browser to apply; kickoff: true sends the Ettore Fit starting prompt instead of message. The reply streams as server-sent events (see the controller doc).