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/Payment/PaymentController.php

@@ -65,11 +65,13 @@ class PaymentController {
 
 			$student = get_userdata( $payment->studentId );
 
-			$groups[ $key ]['total_raw'] += $payment->total();
+			// Show what the student still owes — the amount less any account credit
+			// already applied to this payment.
+			$groups[ $key ]['total_raw'] += $payment->netDue();
 			$groups[ $key ]['rows'][]     = [
 				'id'              => (int) $payment->id,
 				'student'         => $student ? $student->display_name : (string) $payment->studentId,
-				'amount'          => number_format( $payment->amount, 2 ) . ' ' . $payment->currency,
+				'amount'          => number_format( $payment->netDue(), 2 ) . ' ' . $payment->currency,
 				'method'          => $payment->method,
 				'for'             => $payment->registrationType . ' #' . $payment->registrationId,
 				'etransfer_email' => (string) $payment->etransferEmail,