Stream Rally Events

Server-Sent Events stream that forwards raw activity_result/team events.

Unlike /scoreboard/stream (which only signals "leaderboard changed"), this stream forwards each event's type and JSON payload so callers can react to the specific team/activity involved — e.g. staff-evaluation and team-progress pages invalidating only their own affected queries instead of refetching on every unrelated event.

Gated on show_live_leaderboard like the scoreboard itself: the payloads it forwards carry the same score changes, so leaving it open would hand out live standings by another name.