Server-Sent Events stream for a board (realtime)

Opens a long-lived text/event-stream of card.created / card.updated / card.deleted (and checklist) events scoped to one project, plus a : ping comment every 30s to keep the connection warm. Also carries the caller's own notification.created / notification.updated events (each {"unseen": <n>}) - a board tab subscribes to both the board topic and the caller's per-user notification topic on the same stream, so the notification bell stays live without a second connection. See GET /me/events for the per-user stream on pages with no board open. Token-authenticated callers can subscribe, but typical API workflows poll the REST endpoints - the stream exists primarily for the SPA.