Calculate the 30-day readmission rate per department. A readmission is when the same patient visits the same department within 30 days of a completed visit. Use CTEs to find readmissions, then compute the rate as readmissions / total completed visits * 100. Return department name, total_completed, readmission_count, and readmission_rate (rounded to 1 decimal). Order by readmission_rate descending.