Get league activity feed

Returns recent league activity entries. Requires a personal or developer API key whose owner is currently a league ADMIN of the league in the path, with leagues.league_admin:read access on the key. These admin reads no longer require a paid developer plan; they stay authority-gated (current league ADMIN) and rate-limited by the league owner's plan. League API WRITE routes still require the leagueOperatorApi feature (api_pro or higher) on the league owner's plan.