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/GroupClass/EnrollmentRepositoryTest.php

@@ -110,6 +110,35 @@ class EnrollmentRepositoryTest extends TestCase
         self::assertInstanceOf(Enrollment::class, $all[0]);
     }
 
+    public function testFindActiveByOfferingReturnsActiveEnrolmentsOldestFirst(): void
+    {
+        $this->db->shouldReceive('prepare')
+            ->once()
+            ->with(
+                Mockery::pattern('/offering_id = %d AND status = %s ORDER BY id ASC/'),
+                'wp_us_group_enrollments',
+                9,
+                Enrollment::STATUS_ACTIVE
+            )
+            ->andReturn('SELECT ...');
+
+        $this->db->shouldReceive('get_results')->andReturn([
+            (object) [
+                'id'            => '7',
+                'offering_id'   => '9',
+                'student_id'    => '5',
+                'instructor_id' => '3',
+                'status'        => Enrollment::STATUS_ACTIVE,
+                'payment_id'    => null,
+            ],
+        ]);
+
+        $found = $this->repo->findActiveByOffering(9);
+
+        self::assertCount(1, $found);
+        self::assertSame(7, $found[0]->id);
+    }
+
     public function testFindActiveByBillingModesJoinsOfferingAndFiltersModes(): void
     {
         $this->db->shouldReceive('prepare')