The marketing team wants to find days when a user was active on both the website and the mobile app.
The web_events and mobile_events tables record activity timestamps in UTC. A user qualifies for a particular calendar day only if they have at least one event on each platform on that same UTC date. Activity on consecutive dates does not count, even when the events are only minutes apart.
Return one row for each qualifying user and date, regardless of how many events they generated that day.
Ordering: Sort by user_id, then active_day, both ascending.