Rolling 7 day average DAU

Return one row for every calendar date from the first event date to the last event date, including days where nobody did anything.

• dau is the number of distinct users active that day (0 on empty days)
• avg_7d is the average dau over that day and the 6 days before it, rounded to 2 decimal places. Empty days count as 0 in the average.
• for the first 6 dates there isn't a full week yet, so avg_7d is NULL there