SQL: Active Users Via CTE Chain

> This problem is paper-only - see explanation for the canonical SQL solution. A CTE (Common Table Expression) lets you name an intermediate result and reference it later. CTE chains improve readability of multi-step queries. Given `Sessions(user_id, session_date)` rows (each row = one session a user had on a date), return user_ids who had **at least 3 sessions in the last 30 days** of the input. **For the auto-grader:** implement `activeUsers(sessions, asOfDate)` where `asOfDate` is the inclusive upper bound (ISO date). Look back exactly 29 days before `asOfDate` (so 30 days inclusive). Return ids sorted ascending.