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/Schema.php

@@ -159,6 +159,7 @@ class Schema {
                 status                   VARCHAR(20)     NOT NULL DEFAULT 'pending',
                 tax_rate                 DECIMAL(5,2)    NOT NULL DEFAULT 0,
                 tax_amount               DECIMAL(10,2)   NOT NULL DEFAULT 0,
+                credit_applied           DECIMAL(10,2)   NOT NULL DEFAULT 0,
                 due_date                 DATE            DEFAULT NULL,
                 period_key               VARCHAR(20)     DEFAULT NULL,
                 notice_batch             VARCHAR(32)     DEFAULT NULL,
@@ -175,6 +176,24 @@ class Schema {
                 KEY status (status)
             ) {$charset};",
 
+			"CREATE TABLE {$prefix}us_credits (
+                id                BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
+                student_id        BIGINT UNSIGNED NOT NULL,
+                amount            DECIMAL(10,2)   NOT NULL DEFAULT 0,
+                remaining         DECIMAL(10,2)   NOT NULL DEFAULT 0,
+                currency          VARCHAR(3)      NOT NULL DEFAULT 'CAD',
+                source_payment_id BIGINT UNSIGNED DEFAULT NULL,
+                source_lesson_id  BIGINT UNSIGNED DEFAULT NULL,
+                reason            VARCHAR(191)    DEFAULT NULL,
+                status            VARCHAR(20)     NOT NULL DEFAULT 'available',
+                created_at        DATETIME        NOT NULL,
+                updated_at        DATETIME        DEFAULT NULL,
+                PRIMARY KEY (id),
+                KEY student_id (student_id),
+                KEY status (status),
+                KEY source_lesson_id (source_lesson_id)
+            ) {$charset};",
+
 			"CREATE TABLE {$prefix}us_group_enrollments (
                 id            BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
                 offering_id   BIGINT UNSIGNED NOT NULL,