Daily active users for the last N days, split into new vs returning.

The reference consumer of core.active_users, which is now the single platform definition (audit of 28 Sep 2026, Nancy's "one definition of an active user"). The twelve-source union, the team-only pool, the New York day and the returning rule all live there; this endpoint is a cache and a shape.

Sync (def) + 5min cache, the union is heavy, the data moves on day timescales, and the dashboard polls this on every refresh.