Subscription Expiry Date Calculation

A subscription analytics platform needs to compute the effective end date for every subscription record. This `expiry_dt` field is consumed downstream by churn models, renewal reminder pipelines, and cohort analysis jobs. You are given a `sales` table that contains both purchase and refund events for subscriptions. Each purchase record has a `plan` type and may be followed by a refund row for the same `user_id` and `product_id`. **Plan Duration:** - `monthly` plan: expiry = `transaction_dt + 1 month` - `yearly` plan: expiry = `transaction_dt + 1 year` **Refund Override Rule:** - If a purchase has a subsequent refund (same user + product, next transaction is `category = 'refund'`), the `expiry_dt` is the refund's `transaction_dt` instead of the plan's natural end date. **Output:** Return only purchase rows (not refund rows) with columns `user_id`, `product_id`, `transaction_dt`, `price`, `plan`, and `expiry_dt`. Order by `user_id`, `product_id`, `transaction_dt`.