List active Polymarket events (Gamma API)

Paginated by opaque cursor — pass the previous response's `nextCursor` as `after` for the next page; `nextCursor === null` signals end-of-stream. `limit` is clamped to `[1, 200]` (default 50). Cached 30s per (offset, limit, active) tuple. Cursor uses the same base64url encoding as `/v1/events` so customers wrapping pagination once cover both endpoints.