fix(coderd/database): aggregate user engagement statistics by interval (#16150)

This PR addresses the TODO comment here:

https://github.com/coder/coder/pull/16134/files#diff-1844f895bb005f036da11d800fe2a76b54bfddd481c5d8cb15f210c64679caa5R47

The backend now backfills entries for dates with no status changes.

coderd/database/queries/insights.sql

@@ -789,25 +789,11 @@ WITH
 	-- dates_of_interest defines all points in time that are relevant to the query.
 	-- It includes the start_time, all status changes, all deletions, and the end_time.
 dates_of_interest AS (
-	SELECT @start_time::timestamptz AS date
-
-	UNION
-
-    SELECT DISTINCT changed_at AS date
-    FROM user_status_changes
-    WHERE changed_at > @start_time::timestamptz
-        AND changed_at < @end_time::timestamptz
-
-	UNION
-
-	SELECT DISTINCT deleted_at AS date
-	FROM user_deleted
-	WHERE deleted_at > @start_time::timestamptz
-		AND deleted_at < @end_time::timestamptz
-
-	UNION
-
-	SELECT @end_time::timestamptz AS date
+	SELECT date FROM generate_series(
+		@start_time::timestamptz,
+		@end_time::timestamptz,
+		(CASE WHEN @interval::int <= 0 THEN 3600 * 24 ELSE @interval::int END || ' seconds')::interval
+	) AS date
 ),
 	-- latest_status_before_range defines the status of each user before the start_time.
 	-- We do not include users who were deleted before the start_time. We use this to ensure that
@@ -883,7 +869,7 @@ ranked_status_change_per_user_per_date AS (
 	LEFT JOIN relevant_status_changes rsc1 ON rsc1.changed_at <= d.date
 )
 SELECT
-	rscpupd.date,
+	rscpupd.date::timestamptz AS date,
 	statuses.new_status AS status,
 	COUNT(rscpupd.user_id) FILTER (
 		WHERE rscpupd.rn = 1