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]>

CHANGELOG.md

@@ -15,6 +15,7 @@ each change under the current top section as you work.
 
 ### Added
 - Offerings can now bill on a schedule: **weekly** (a pending payment 24 hours before each lesson) or **monthly** (one payment on the 1st for that month's lessons), alongside the existing one-time and full-term modes. Applies to both private lessons and group classes. A daily job generates due payments, and each student receives one consolidated itemised email per scan; batched payments share a reference so the admin Payments queue groups them with a lump-sum total for e-transfer reconciliation. Cancelling a lesson never voids a scheduled payment.
+- Cancelling a lesson that was **already paid for** now credits the student that money instead of leaving it as a manual refund. The credit is one lesson's share of what they paid — the whole amount for a single lesson, or a per-lesson slice of a monthly charge or a full-term series. The daily billing scan automatically applies any available credit against a student's upcoming weekly/monthly charges before emailing their notice, which shows the credit applied and the reduced total due; a charge fully covered by credit is settled and leaves the admin Payments queue. A student's outstanding credit balance is shown on their **student detail** page in the studio admin. Still-pending (unpaid) payments continue to be voided on cancellation as before.
 - Group classes now carry an **enrolment deadline** the instructor sets on the offering. It defaults to the first day of the class, and once it passes students can no longer enrol — the enrolment page shows the class as closed and the API rejects late enrolments. While enrolment is open, each class card shows an "Enrol by" date.
 - Instructors can add students to any group class by hand from its details page (**Add students directly**), which now appears for public classes too, not just invite-only ones. This bypasses the enrolment deadline and capacity, so a student can be enrolled as a **late enrolment** after the class has closed to self-enrolment.
 - Studio admins and instructors can open a **lesson detail view** from the Scheduler and My Lessons lists, showing the offering booked, the policy versions the student accepted (with acceptance time and IP), and their intake answers. On My Lessons an instructor may only open their own lessons; the studio Scheduler may open any.