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/OfferingEndpointTest.php

@@ -7,6 +7,7 @@ use Brain\Monkey\Functions;
 use Mockery;
 use Unsupervised\Schedular\Auth\RoleManager;
 use Unsupervised\Schedular\GroupClass\GroupAccessRepository;
+use Unsupervised\Schedular\Offering\BillingModeReconciler;
 use Unsupervised\Schedular\Offering\Offering;
 use Unsupervised\Schedular\Offering\OfferingEndpoint;
 use Unsupervised\Schedular\Offering\OfferingRepository;
@@ -16,6 +17,7 @@ class OfferingEndpointTest extends TestCase
 {
     private OfferingRepository&Mockery\MockInterface $repository;
     private GroupAccessRepository&Mockery\MockInterface $access;
+    private BillingModeReconciler&Mockery\MockInterface $billingModeReconciler;
     private OfferingEndpoint $endpoint;
 
     protected function setUp(): void
@@ -27,7 +29,9 @@ class OfferingEndpointTest extends TestCase
 
         $this->repository = Mockery::mock(OfferingRepository::class);
         $this->access     = Mockery::mock(GroupAccessRepository::class);
-        $this->endpoint   = new OfferingEndpoint($this->repository, $this->access);
+        $this->billingModeReconciler = Mockery::mock(BillingModeReconciler::class);
+        $this->billingModeReconciler->shouldReceive('reconcile')->andReturn(0)->byDefault();
+        $this->endpoint   = new OfferingEndpoint($this->repository, $this->access, $this->billingModeReconciler);
     }
 
     private function group(int $id, string $access): Offering