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/Payment/ScheduledBillingRunnerTest.php

@@ -40,6 +40,8 @@ class ScheduledBillingRunnerTest extends TestCase
         $this->enrollments->shouldReceive('findActiveByBillingModes')->andReturn([])->byDefault();
         $this->mailer->shouldReceive('send')->andReturn(true)->byDefault();
         $this->payments->shouldReceive('assignNoticeBatch')->byDefault();
+        // No account credit unless a test says otherwise.
+        $this->payments->shouldReceive('applyCredits')->andReturn([])->byDefault();
 
         Functions\when('wp_generate_uuid4')->justReturn('abcdef12-3456-7890-abcd-ef1234567890');
 
@@ -238,7 +240,52 @@ class ScheduledBillingRunnerTest extends TestCase
             ->with(Mockery::on(static fn (array $ids): bool => count($ids) === 2), Mockery::type('string'));
         $this->mailer->shouldReceive('send')
             ->once()
-            ->with(Mockery::type(\WP_User::class), Mockery::on(static fn (array $items): bool => count($items) === 2), Mockery::type('string'));
+            ->with(Mockery::type(\WP_User::class), Mockery::on(static fn (array $items): bool => count($items) === 2), Mockery::type('string'), 0.0);
+
+        $this->runner->run();
+    }
+
+    public function testAppliesAccountCreditToTheRun(): void
+    {
+        $this->now('2026-07-15 09:00:00');
+        $this->bookings->shouldReceive('findUnbilledScheduledLessons')
+            ->andReturn([ $this->lessonRow(101, Offering::BILLING_WEEKLY, '2026-07-15 18:00:00', 35.0) ]);
+
+        $payment = $this->pending(500, '2026-07-14');
+        $this->payments->shouldReceive('createForRegistration')->once()->andReturn($payment);
+
+        // Student holds $20 credit, applied to the one $35 charge — still $15 owing,
+        // so the payment stays in the notice batch and the notice quotes the credit.
+        $this->payments->shouldReceive('applyCredits')
+            ->once()
+            ->with(5, Mockery::on(static fn (array $p): bool => count($p) === 1))
+            ->andReturn([500 => 20.0]);
+        $this->payments->shouldReceive('assignNoticeBatch')
+            ->once()
+            ->with([500], Mockery::type('string'));
+        $this->mailer->shouldReceive('send')
+            ->once()
+            ->with(Mockery::type(\WP_User::class), Mockery::type('array'), Mockery::type('string'), 20.0);
+
+        $this->runner->run();
+    }
+
+    public function testCreditFullyCoveringAChargeLeavesItOutOfTheBatch(): void
+    {
+        $this->now('2026-07-15 09:00:00');
+        $this->bookings->shouldReceive('findUnbilledScheduledLessons')
+            ->andReturn([ $this->lessonRow(101, Offering::BILLING_WEEKLY, '2026-07-15 18:00:00', 35.0) ]);
+
+        $payment = $this->pending(500, '2026-07-14');
+        $this->payments->shouldReceive('createForRegistration')->once()->andReturn($payment);
+
+        // Credit covers the whole $35 charge: nothing owing, so no reconciliation
+        // batch and no reference on the (zero-balance) notice.
+        $this->payments->shouldReceive('applyCredits')->once()->andReturn([500 => 35.0]);
+        $this->payments->shouldReceive('assignNoticeBatch')->once()->with([], '');
+        $this->mailer->shouldReceive('send')
+            ->once()
+            ->with(Mockery::type(\WP_User::class), Mockery::type('array'), '', 35.0);
 
         $this->runner->run();
     }