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;
 	}

src/Auth/StudentController.php

@@ -149,11 +149,24 @@ class StudentController {
 		$registrationInfo = $this->history->registrationInfo( (int) $student->ID );
 		$intake           = $this->history->intakeAnswers( (int) $student->ID );
 		$payments         = $canBilling ? $this->history->payments( (int) $student->ID ) : [];
+		$credits          = $canBilling ? $this->history->credits( (int) $student->ID ) : [];
+		$creditBalance    = $canBilling ? $this->history->creditBalance( (int) $student->ID ) : 0.0;
+		$creditCurrency   = $this->creditCurrency( $credits );
 
 		$backUrl = admin_url( 'admin.php?page=us-students' );
 		include USC_PLUGIN_DIR . 'templates/admin/student-detail.php';
 	}
 
+	/**
+	 * Currency to label the credit balance with — taken from the student's credits
+	 * (they share a currency in practice), defaulting to CAD when they have none.
+	 *
+	 * @param list<array{created_at: string, amount: float, remaining: float, currency: string, reason: string, status: string}> $credits
+	 */
+	private function creditCurrency( array $credits ): string {
+		return [] !== $credits ? (string) $credits[0]['currency'] : 'CAD';
+	}
+
 	/**
 	 * Build a display row for a lesson (slot time, offering, instructor, status).
 	 *

src/Auth/StudentHistory.php

@@ -3,6 +3,8 @@ declare(strict_types=1);
 
 namespace Unsupervised\Schedular\Auth;
 
+use Unsupervised\Schedular\Payment\Credit;
+use Unsupervised\Schedular\Payment\CreditRepository;
 use Unsupervised\Schedular\Payment\Payment;
 use Unsupervised\Schedular\Payment\PaymentRepository;
 use Unsupervised\Schedular\Policy\AcceptanceRepository;
@@ -27,6 +29,7 @@ class StudentHistory {
 		private AnswerRepository $answers,
 		private QuestionRepository $questions,
 		private PaymentRepository $payments,
+		private CreditRepository $credits,
 	) {}
 
 	/**
@@ -129,6 +132,34 @@ class StudentHistory {
 		);
 	}
 
+	/**
+	 * The student's total unused credit balance (from cancelled paid lessons),
+	 * applied automatically against future scheduled-billing charges.
+	 */
+	public function creditBalance( int $studentId ): float {
+		return $this->credits->availableBalance( $studentId );
+	}
+
+	/**
+	 * Every credit the student has been issued, newest first, with the amount, what
+	 * remains, and its state.
+	 *
+	 * @return list<array{created_at: string, amount: float, remaining: float, currency: string, reason: string, status: string}>
+	 */
+	public function credits( int $studentId ): array {
+		return array_map(
+			static fn( Credit $credit ): array => [
+				'created_at' => $credit->createdAt ?? '',
+				'amount'     => $credit->amount,
+				'remaining'  => $credit->remaining,
+				'currency'   => $credit->currency,
+				'reason'     => $credit->reason ?? '—',
+				'status'     => $credit->status,
+			],
+			$this->credits->findByStudent( $studentId )
+		);
+	}
+
 	/**
 	 * Human label for a polymorphic registration target.
 	 */