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.