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

src/GroupClass/EnrollmentRepository.php

@@ -82,6 +82,24 @@ class EnrollmentRepository {
 		return $count > 0;
 	}
 
+	/**
+	 * Active enrolments in one offering, oldest first.
+	 *
+	 * @return list<Enrollment>
+	 */
+	public function findActiveByOffering( int $offeringId ): array {
+		$rows = $this->db->get_results(
+			$this->db->prepare(
+				'SELECT * FROM %i WHERE offering_id = %d AND status = %s ORDER BY id ASC',
+				$this->table,
+				$offeringId,
+				Enrollment::STATUS_ACTIVE
+			)
+		);
+
+		return array_map( Enrollment::fromRow( ... ), $rows ?? [] );
+	}
+
 	/**
 	 * A student's enrolments, newest first.
 	 *