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

tests/Unit/Offering/OfferingControllerTest.php

@@ -5,6 +5,7 @@ namespace Unsupervised\Schedular\Tests\Unit\Offering;
 
 use Brain\Monkey\Functions;
 use Mockery;
+use Unsupervised\Schedular\Offering\BillingModeReconciler;
 use Unsupervised\Schedular\Offering\ClassSlotReconciler;
 use Unsupervised\Schedular\Offering\Offering;
 use Unsupervised\Schedular\Offering\OfferingController;
@@ -15,6 +16,7 @@ class OfferingControllerTest extends TestCase
 {
     private OfferingRepository&Mockery\MockInterface $repository;
     private ClassSlotReconciler&Mockery\MockInterface $reconciler;
+    private BillingModeReconciler&Mockery\MockInterface $billingModeReconciler;
     private OfferingController $controller;
 
     protected function setUp(): void
@@ -24,7 +26,9 @@ class OfferingControllerTest extends TestCase
         $this->repository = Mockery::mock(OfferingRepository::class);
         $this->reconciler = Mockery::mock(ClassSlotReconciler::class);
         $this->reconciler->shouldReceive('reconcile')->andReturn(['removed' => 0, 'conflicts' => []])->byDefault();
-        $this->controller = new OfferingController($this->repository, $this->reconciler);
+        $this->billingModeReconciler = Mockery::mock(BillingModeReconciler::class);
+        $this->billingModeReconciler->shouldReceive('reconcile')->andReturn(0)->byDefault();
+        $this->controller = new OfferingController($this->repository, $this->reconciler, $this->billingModeReconciler);
 
         $_POST = [];
         $_GET  = [];