Rolling 7-day distinct active users

Table activity:

A user can have several rows on the same day.

For every calendar day from the first to the last day in the table, return the number of distinct users who were active on that day or any of the 6 days before it (a 7-day window ending on that day). Days with nobody active in the window have count 0.

Output columns: day, active_users. Order by day.

CREATE TABLE activity (
  user_id int NOT NULL,
  activity_date date NOT NULL
);