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/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).
 	 *