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/PaymentDueMailerTest.php

@@ -74,6 +74,46 @@ class PaymentDueMailerTest extends TestCase
         self::assertTrue((new PaymentDueMailer())->send($this->student('[email protected]'), $items, 'REF12345'));
     }
 
+    public function testCreditReducesTheTotalDue(): void
+    {
+        Functions\expect('wp_mail')
+            ->once()
+            ->with(
+                '[email protected]',
+                Mockery::type('string'),
+                Mockery::on(static function (string $body): bool {
+                    // Line shows the full 35.00; credit line shows -20.00; total due 15.00.
+                    return str_contains($body, '35.00')
+                        && str_contains($body, '-CAD 20.00')
+                        && str_contains($body, 'Total due: CAD 15.00');
+                })
+            )
+            ->andReturn(true);
+
+        $items = [[ 'label' => 'Piano', 'amount' => 35.0, 'currency' => 'CAD', 'due_date' => '2026-07-15', 'etransfer_email' => '[email protected]' ]];
+
+        self::assertTrue((new PaymentDueMailer())->send($this->student('[email protected]'), $items, 'REF1', 20.0));
+    }
+
+    public function testCreditCoveringEverythingLeavesZeroDueAndNoEtransferLine(): void
+    {
+        Functions\expect('wp_mail')
+            ->once()
+            ->with(
+                '[email protected]',
+                Mockery::type('string'),
+                Mockery::on(static function (string $body): bool {
+                    return str_contains($body, 'Total due: CAD 0.00')
+                        && ! str_contains($body, '[email protected]');
+                })
+            )
+            ->andReturn(true);
+
+        $items = [[ 'label' => 'Piano', 'amount' => 35.0, 'currency' => 'CAD', 'due_date' => '2026-07-15', 'etransfer_email' => '[email protected]' ]];
+
+        self::assertTrue((new PaymentDueMailer())->send($this->student('[email protected]'), $items, '', 35.0));
+    }
+
     public function testIncludesEtransferDestination(): void
     {
         Functions\expect('wp_mail')