Get Overview Cohort Event Rates

All pinned cohort grids in one round-trip — the batched form of the singular /overview/cohort-event-rate GET, fanned out per target so the engagement section issues one POST instead of one GET per card. Each target runs the same cache-backed GetOverviewCohortEventRateUseCase on its own session (concurrent reads); one target failing (404 outcome, 400 statistic mismatch) degrades to a per-outcome error entry, never a batch 500. Rate- limited per (org, user) like the singular route + the sibling batched /global-lift/measure fan-out; the Overview drives it on mount and on outcome/window/statistic change.