List every Kalshi series

Returns every series Kalshi exposes (crypto, sports, politics, weather, etc.). Cached 60s; safe to poll. Note: the upstream `/series` payload is ~10 MB and ignores `limit` — the first call after a cache miss is slow (up to 8s); subsequent callers in the 60s window get the cached snapshot. Use `/series/{ticker}/markets` for the per-series drill-down.