Add async PAPI leaderboard cache and placeholders (#5301)
Fixes #5171 Fixes #4800 Fixes #4291
Placeholders are served from an async snapshot cache refreshed in bulk per backend interval; includes SQL per-skill leaderboard indexes with a query rewrite, ghost-row filtering shared by /mctop and /mcrank, FlatFile leaderboard generation-swap thread safety, a routing pin test for the PAPI token seam, and placeholder token names built with Locale.ENGLISH. (commit: 0dcf35b)