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]>

docs/features/student-administration.md

@@ -33,6 +33,10 @@ No new tables. The views are composed from existing data:
     and when it was accepted.
   - **Intake answers** — every registration-question answer, newest first:
     question label, answer, and the registration it was given for.
+  - **Account credit** (`manage_billing` only) — the student's available credit
+    balance plus every credit (date, reason, amount, remaining, status). Credit
+    comes from cancelled paid lessons and is applied automatically to upcoming
+    scheduled billing. See `credits.md`.
   - **Payment history** (`manage_billing` only) — every payment, newest first:
     date, context, method, status, subtotal, HST, total, and receipt number.
 
@@ -44,7 +48,8 @@ All actions are nonce-protected POSTs handled on the detail page:
 - **Cancel lesson** — on any non-cancelled upcoming lesson. Uses the same path
   as student-initiated cancellation: the lesson is marked `cancelled`, the
   availability slot is freed for rebooking, and a still-pending payment is
-  voided. Paid lessons keep their payment — refunds stay a manual decision (#72).
+  voided. A paid lesson is credited back to the student's account (see
+  `credits.md`) rather than refunded.
 - **Withdraw** — on an active group-class enrolment: marked `cancelled` (freeing
   its capacity seat), with the same pending-payment voiding.