Polymarket global leaderboard (Data API passthrough)

Verbatim passthrough of the official Polymarket data-api `/v1/leaderboard` — the exact upstream JSON array (rank, proxyWallet, userName, vol, pnl, …) is returned as `native`. Cached server-side ~60s per param tuple, so clients need not hit Polymarket directly (QF-10888 #5). `window` is one of 1d/7d/30d/all (default `all`) — Artery maps it to the upstream `timePeriod` (1d→DAY / 7d→WEEK / 30d→MONTH / all→ALL) for BOTH the paged board and the per-user form; upstream ONLY recognizes `timePeriod`, so passing `window` unmapped silently returns the DAY board (QF-10947). `orderBy` pnl (default) or vol; `limit` defaults to 25 and is hard-capped at 50 by Polymarket; `offset` pages through ranks. Pass `user=<proxyWallet>` to fetch that ONE wallet's row instead of the paged board (QF-10938 #1) — orderBy/limit/offset are then omitted (irrelevant for a single row).