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/Plugin.php

@@ -18,6 +18,7 @@ use Unsupervised\Schedular\GroupClass\GroupAccessRepository;
 use Unsupervised\Schedular\GroupClass\GroupClassPage;
 use Unsupervised\Schedular\Offering\OfferingRepository;
 use Unsupervised\Schedular\Payment\BillingMethodResolver;
+use Unsupervised\Schedular\Payment\CreditRepository;
 use Unsupervised\Schedular\Payment\PaymentRepository;
 use Unsupervised\Schedular\Payment\PaymentDueMailer;
 use Unsupervised\Schedular\Payment\PaymentService;
@@ -65,10 +66,11 @@ class Plugin {
 		$registrationGate = new RegistrationGate( $questions, $answers, $policies, $policyVersions, $acceptances );
 
 		$paymentRepo    = new PaymentRepository( $wpdb );
+		$creditRepo     = new CreditRepository( $wpdb );
 		$settings       = new StudioSettings();
 		$resolver       = new BillingMethodResolver( $settings );
 		$stripe         = new StripeGateway( $settings );
-		$paymentService = new PaymentService( $paymentRepo, $resolver, new ReceiptMailer(), $bookings, $enrollments, $settings, $stripe );
+		$paymentService = new PaymentService( $paymentRepo, $resolver, new ReceiptMailer(), $bookings, $enrollments, $settings, $stripe, $creditRepo );
 
 		// The shortcode and block wrappers share the same page objects so
 		// front-end output is identical whichever way a page embeds them.
@@ -85,7 +87,7 @@ class Plugin {
 		( new RoleManager() )->register();
 		( new RegistrationLoginGate() )->register();
 		( new EmailConfirmationHandler( $settings, $registrationMailer ) )->register();
-		( new AdminMenu( $availability, $bookings, $offerings, $questions, $answers, $policies, $policyVersions, $policyService, $acceptances, $invites, $enrollments, $groupAccess, $settings, $paymentRepo, $paymentService, $resolver, $registrationMailer ) )->register();
+		( new AdminMenu( $availability, $bookings, $offerings, $questions, $answers, $policies, $policyVersions, $policyService, $acceptances, $invites, $enrollments, $groupAccess, $settings, $paymentRepo, $paymentService, $resolver, $registrationMailer, $creditRepo ) )->register();
 		( new RestRegistrar( $availability, $bookings, $offerings, $questions, $policies, $policyVersions, $policyService, $registrationGate, $enrollments, $groupAccess, $paymentService ) )->register();
 		( new ShortcodeRegistrar( $bookingPage, $loginPage, $registrationPage, $groupClassPage ) )->register();
 		( new BlockRegistrar( $bookingPage, $loginPage, $registrationPage, $groupClassPage ) )->register();