Add changelog entries for the credit fix and student-page display

Recorded under the current 1.5.8 section; no version bump.

Co-authored-by: anthropic/claude-opus-4-8

CHANGELOG.md

@@ -13,7 +13,11 @@ each change under the current top section as you work.
 
 ## [1.5.8]
 
+### Added
+- **A student's account credit balance now shows at the top of their detail page.** Credit from a cancelled paid lesson was already recorded and listed further down the page, but you had to scroll to the Account credit section to find out a student was owed anything. When there is a balance to report it now appears up top the moment you open the page, so you can see at a glance that this student's future billing will be offset — and, for a child, that the balance sits on their guardian's account. The full breakdown of where the credit came from stays where it was.
+
 ### Fixed
+- **Rebooking a cancelled paid lesson in the same month no longer charges the family twice.** Cancelling a paid lesson credits the account for it, and that credit is meant to cover the next lesson booked in its place. But a lesson booked back into a month already billed is charged there and then, and that charge skipped the step where credit is applied — so the family was billed in full for the replacement while the credit for the cancelled lesson sat unused, in effect paying twice for the one slot. Account credit is now applied to a charge raised at booking, so the credit settles the rebooking the same way it settles a scheduled charge; a lesson fully covered by credit is confirmed with nothing left to pay.
 - **A student is no longer emailed the same "Payment due" notice twice.** The daily billing scan runs whenever the site gets traffic, and on a busy day two copies of it could end up running at the same time. Neither knew about the other, so each would send its own notice for the same charge — one payment on the books, but the family saw two identical requests to pay and reasonably read it as being billed twice. Each payment is now stamped the moment its notice goes out, and a second run that reaches the same payment sees the stamp and stays quiet, so exactly one notice is sent no matter how the scan is triggered. Payments already noticed before this update are marked as such on upgrade, so nobody gets a fresh round of reminders for charges they were already told about.
 - **Switching a group class to monthly billing no longer charges students who already paid up front a second time.** When a class was set up to be paid once at sign-up and later changed to bill monthly, the daily scan did not recognise the payment already taken at enrolment — it carried no billing month — and raised a fresh charge for the current month on top of it. Families who had already paid were billed again, sometimes for a month they had covered. Changing a class to monthly now marks each enrolled student's up-front payment as covering the current month, so the scan bills them from the following month on and never doubles up on the month already paid. (Enrolments made after the switch, and classes that were always monthly, were never affected.)