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/Payment/PaymentRepository.php

@@ -25,6 +25,7 @@ class PaymentRepository {
 				'status'                   => $payment->status,
 				'tax_rate'                 => $payment->taxRate,
 				'tax_amount'               => $payment->taxAmount,
+				'credit_applied'           => $payment->creditApplied,
 				'due_date'                 => $payment->dueDate,
 				'period_key'               => $payment->periodKey,
 				'notice_batch'             => $payment->noticeBatch,
@@ -35,7 +36,7 @@ class PaymentRepository {
 				'paid_at'                  => $payment->paidAt,
 				'created_at'               => current_time( 'mysql' ),
 			],
-			[ '%d', '%d', '%s', '%d', '%f', '%s', '%s', '%s', '%f', '%f', '%s', '%s', '%s', '%s', '%s', '%s', '%s', '%s', '%s' ]
+			[ '%d', '%d', '%s', '%d', '%f', '%s', '%s', '%s', '%f', '%f', '%f', '%s', '%s', '%s', '%s', '%s', '%s', '%s', '%s', '%s' ]
 		);
 
 		return $this->db->insert_id;
@@ -77,6 +78,22 @@ class PaymentRepository {
 		);
 	}
 
+	/**
+	 * Add to the account credit applied against a payment, reducing what the student
+	 * still owes on it (`Payment::netDue()`). Accumulates, so a second application
+	 * adds to the first.
+	 */
+	public function addCreditApplied( int $id, float $amount ): bool {
+		$sql = $this->db->prepare(
+			'UPDATE %i SET credit_applied = credit_applied + %f WHERE id = %d',
+			$this->table,
+			$amount,
+			$id
+		);
+
+		return null !== $sql && false !== $this->db->query( $sql );
+	}
+
 	/**
 	 * Set a payment's tax rate and recompute the tax amount from its subtotal.
 	 */