Compute multi-account actives and unread coverage

We are given two relations. The first, notifications, stores account-level (per-userid) events. The second, dim_circle_people, maps each account to the person who owns it and records activity metadata for that account. The reporting window is fixed to the whole of August 2025, treated as UTC. Filter that month with notifications.ds (when both ds and event_time are populated, ds decides the month and event_time is auxiliary). An account counts as active in the window when its…