Get Engagement Summary

Per-category engagement rollup powering the dashboard summary cards.

Returns one row per category in EVENT_CATEGORY_MAP plus a top-events breakdown and a 7/14/30-day sparkline (daily counts) for each. Counts are sourced from user_activity_sessions.activities_performed (the same JSONB dict that retention/activation endpoints already use), so any event a feature emits via trackFeatureUse(...) shows up automatically.

The categories are coarse on purpose — fine-grained per-event detail is on the Activity Monitoring page. The point of this endpoint is the "is this whole feature area being used" headline number.