Stream real-time conversation events via SSE

Server-Sent Events stream of messaging-channel events scoped to a single conversation (SMS or WhatsApp). Each frame is a JSON envelope: event_type, conversation_id, timestamp, and a payload object whose shape depends on the event type:

• message.received / message.delivered / message.read / message.failed: payload contains { message_id, direction, text, status, ... }.
• conversation.mode_changed: payload contains { from_mode, to_mode, actor_user_id }.

The stream closes when the client disconnects or the conversation is archived.

Authenticate with the Authorization: Bearer header. Unlike the call transcript stream, the ?token= query-param shim is not applied to this path, so a raw browser EventSource (which cannot set headers) must connect through a proxy that adds the header.