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]>

tests/Unit/Auth/StudentHistoryTest.php

@@ -5,6 +5,8 @@ namespace Unsupervised\Schedular\Tests\Unit\Auth;
 
 use Mockery;
 use Unsupervised\Schedular\Auth\StudentHistory;
+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 StudentHistoryTest extends TestCase
     private AnswerRepository&Mockery\MockInterface $answers;
     private QuestionRepository&Mockery\MockInterface $questions;
     private PaymentRepository&Mockery\MockInterface $payments;
+    private CreditRepository&Mockery\MockInterface $credits;
     private StudentHistory $history;
 
     protected function setUp(): void
@@ -39,6 +42,7 @@ class StudentHistoryTest extends TestCase
         $this->answers        = Mockery::mock(AnswerRepository::class);
         $this->questions      = Mockery::mock(QuestionRepository::class);
         $this->payments       = Mockery::mock(PaymentRepository::class);
+        $this->credits        = Mockery::mock(CreditRepository::class);
 
         $this->history = new StudentHistory(
             $this->acceptances,
@@ -46,7 +50,8 @@ class StudentHistoryTest extends TestCase
             $this->policyVersions,
             $this->answers,
             $this->questions,
-            $this->payments
+            $this->payments,
+            $this->credits
         );
     }
 
@@ -215,4 +220,34 @@ class StudentHistoryTest extends TestCase
         self::assertSame('Enrolment #3', $rows[0]['context']);
         self::assertSame('—', $rows[0]['receipt']);
     }
+
+    public function testCreditBalanceDelegatesToRepository(): void
+    {
+        $this->credits->shouldReceive('availableBalance')->once()->with(5)->andReturn(45.0);
+
+        self::assertSame(45.0, $this->history->creditBalance(5));
+    }
+
+    public function testCreditsBuildDisplayRows(): void
+    {
+        $this->credits->shouldReceive('findByStudent')->once()->with(5)->andReturn([
+            new Credit(5, 33.00, 13.00, 'CAD', 12, 77, 'Credit for cancelled lesson #77', Credit::STATUS_AVAILABLE, '2026-07-01 09:00:00', id: 300),
+        ]);
+
+        $rows = $this->history->credits(5);
+
+        self::assertSame(
+            [
+                [
+                    'created_at' => '2026-07-01 09:00:00',
+                    'amount'     => 33.00,
+                    'remaining'  => 13.00,
+                    'currency'   => 'CAD',
+                    'reason'     => 'Credit for cancelled lesson #77',
+                    'status'     => Credit::STATUS_AVAILABLE,
+                ],
+            ],
+            $rows
+        );
+    }
 }