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/Auth/StudentActions.php

@@ -27,8 +27,9 @@ class StudentActions {
 
 	/**
 	 * Cancel a lesson on the student's behalf: marks it cancelled, frees the
-	 * slot for rebooking, and voids a still-pending payment. Paid lessons keep
-	 * their payment — refunds are a manual, admin-side decision.
+	 * slot for rebooking, and voids a still-pending payment. A paid lesson is
+	 * credited back to the student's account (a per-lesson share of what they
+	 * paid) to offset their future scheduled billing.
 	 */
 	public function cancelLesson( int $lessonId, int $studentId ): bool {
 		$lesson = $this->bookings->findById( $lessonId );
@@ -40,6 +41,7 @@ class StudentActions {
 		$this->bookings->updateStatus( $lessonId, Lesson::STATUS_CANCELLED );
 		$this->availability->release( $lesson->slotId );
 		$this->payments->voidPending( $lesson->paymentId );
+		$this->payments->creditForCancelledLesson( $lesson );
 
 		return true;
 	}