Credit students for cancelled paid lessons

Cancelling a lesson that was already paid for now credits the student
that money instead of leaving it as a manual refund, and the daily
scheduled-billing scan applies any available credit against their due
charges before emailing the notice.

- New us_credits ledger + us_payments.credit_applied column (Payment::netDue).
- PaymentService::creditForCancelledLesson issues a per-lesson share of the
  covering payment's total; wired into all three cancel paths (student
  self-cancel, instructor status update, admin student-detail cancel).
- PaymentService::applyCredits draws credit down FIFO across a run's charges,
  marking a fully-covered charge paid-by-credit; the notice shows the credit
  applied and reduced total, and the admin queue shows net due.
- Student detail page shows a student's credit balance and history.

Ships as part of the unreleased 1.2.0 (same release as scheduled billing).

Tests: composer test (585), composer lint, composer cs all pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/Booking/BookingEndpoint.php

@@ -343,8 +343,9 @@ class BookingEndpoint {
 
 	/**
 	 * Student-initiated cancellation of their own lesson: marks it cancelled,
-	 * frees the slot for rebooking, and voids any still-pending payment. Paid
-	 * lessons keep their payment — refunds are a manual, admin-side decision.
+	 * frees the slot for rebooking, and voids any still-pending payment. A lesson
+	 * already paid for is credited back to the student's account (a per-lesson
+	 * share of the covering payment) to offset their future scheduled billing.
 	 */
 	public function cancel( \WP_REST_Request $request ): \WP_REST_Response|\WP_Error {
 		$id     = absint( Val::int( $request->get_param( 'id' ) ) );
@@ -379,6 +380,7 @@ class BookingEndpoint {
 			$this->bookings->updateStatus( $id, Lesson::STATUS_CANCELLED );
 			$this->availability->release( $lesson->slotId );
 			$this->payments->voidPending( $lesson->paymentId );
+			$this->payments->creditForCancelledLesson( $lesson );
 		}
 
 		return new \WP_REST_Response(
@@ -407,6 +409,7 @@ class BookingEndpoint {
 		if ( Lesson::STATUS_CANCELLED === $status && Lesson::STATUS_CANCELLED !== $lesson->status ) {
 			$this->availability->release( $lesson->slotId );
 			$this->payments->voidPending( $lesson->paymentId );
+			$this->payments->creditForCancelledLesson( $lesson );
 		} elseif ( Lesson::STATUS_CANCELLED === $lesson->status && Lesson::STATUS_CANCELLED !== $status && ! $this->availability->claim( $lesson->slotId ) ) {
 			// Reinstating a cancelled lesson must re-reserve its slot, and
 			// someone else may have booked the freed time in the meantime.