GET /me/statistics caches account buy-and-hold and adds benchmark=async

Account buy-and-hold (benchmarkHistory) is now cached server-side until the next UTC midnight, and it is recomputed the same day when the running count, total capital or first PnL day changes. Pass benchmark=async to GET /api/v1/me/statistics to get an answer right away: while the series is still computing, the response has benchmarkPending: true and an empty benchmarkHistory. Poll until it is false. Without the parameter the call waits for the series as before. MCP get_user_statistics uses the same cache.