Get platform activity time-series

Platform-wide borrow/repay totals bucketed by period, with cumulative outstanding. Every period in the range is returned, including quiet ones: a period with no borrowing or repayment reports zero totals and carries the outstanding position forward. A carried position is valued at its own period end, so netOutstandingUsd can differ from the period before it even when nothing was borrowed or repaid — a change there is not by itself evidence of activity. Aggregate-only — no per-borrower data. No authentication required.