Reconcile up-front charges when a class switches to monthly billing

A student who enrols while a group class is pay-now is charged once at
enrolment, and that charge carries no period_key. When the class is
later switched to monthly, the daily scan — which dedups scheduled
charges by period_key — does not see the up-front charge and bills the
enrolment again for the current month, double-charging students who
had already paid. The differing payer between the two rows (student vs
guardian) was a side effect of guardian links created between the two
charge dates, not the cause.

Switching a group class into monthly now adopts each active enrolment's
up-front charge into the current month (stamping period_key and
due_date) so the scan treats that month as billed and charges from the
next month on. Enrolments with no up-front charge, or already billed
for the month, are left alone; weekly and non-group offerings are not
touched. Wired into both offering-update paths (admin form and REST).

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

CHANGELOG.md

@@ -15,6 +15,7 @@ each change under the current top section as you work.
 
 ### Fixed
 - **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.)
 
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