Average days between orders

For every customer with at least two completed orders, find the average number of days between one completed order and their next one.

• use calendar dates of order_ts, so two orders on the same day are 0 days apart
• "next one" means next in time, not next by order_id
• round avg_days_between to 1 decimal place
• guest orders (no customer_id) don't belong to a customer, leave them out

Example: completed orders on Jan 1, Jan 3 and Jan 4 give gaps of 2 and 1, so the average is 1.5.