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/StudentActionsTest.php

@@ -42,6 +42,10 @@ class StudentActionsTest extends TestCase
         $this->bookings->shouldReceive('updateStatus')->once()->with(12, Lesson::STATUS_CANCELLED)->andReturn(true);
         $this->availability->shouldReceive('release')->once()->with(7)->andReturn(true);
         $this->payments->shouldReceive('voidPending')->once()->with(40);
+        // A paid lesson is credited; the cancelled lesson value object is handed over.
+        $this->payments->shouldReceive('creditForCancelledLesson')
+            ->once()
+            ->with(Mockery::on(static fn (Lesson $l): bool => $l->id === 12 && $l->paymentId === 40));
 
         self::assertTrue($this->actions->cancelLesson(12, 5));
     }