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/credits.md

@@ -0,0 +1,123 @@
+# Feature: Student Credits (cancelled paid lessons)
+
+## Overview
+When a lesson that has **already been paid for** is cancelled, the student is
+credited the amount they paid for *that lesson*. The credit sits on their account
+and is automatically applied against their future scheduled-billing charges
+(weekly / monthly) before they are asked to pay — so a cancelled-and-paid lesson
+becomes money toward the next one rather than a manual refund.
+
+This complements — it does not replace — the existing cancellation behaviour: a
+still-**pending** payment is voided (`PaymentService::voidPending`), and only a
+**paid** payment produces a credit.
+
+## Credit amount — one lesson's share
+The credit is one lesson's share of the covering payment's **total (including
+tax)**:
+
+| Covering payment | Lessons it covers | Credit on cancelling one |
+|------------------|-------------------|--------------------------|
+| Single booking (one-time / full-term single) | 1 | the whole total |
+| Weekly **scheduled** lesson | 1 (one payment per lesson) | the whole total |
+| Monthly **scheduled** charge | N lessons that month | `total ÷ N` |
+| Weekly reservation **series** paid upfront (full-term) | the whole series | `total ÷ series size` |
+
+The divisor is resolved in `PaymentService::coveredLessonCount`: a weekly series
+paid upfront (an *unscheduled* payment on a lesson that has a `series_id`) divides
+by the series size (`BookingRepository::countBySeries`); every other case divides
+by how many lessons point at the payment (`BookingRepository::countByPaymentId`),
+which is 1 for a single or weekly-scheduled lesson and N for a monthly charge.
+
+The original payment is **left untouched** — the studio keeps the money it
+collected; the credit is a forward-looking liability offset against future
+billing, never a refund of past revenue.
+
+### Guards
+- Only a **paid** payment credits; an unpaid/pending one is voided instead.
+- A lesson is credited **once** — `CreditRepository::existsForLesson` blocks a
+  second credit if the same lesson is cancelled again after being reinstated.
+- A non-anchor lesson in a series (no `payment_id` of its own) is credited through
+  the series anchor's payment.
+
+## Applying credit at billing time
+The daily scan (`Payment\ScheduledBillingRunner`) generates each student's due
+payments, then — before sending the notice — applies their available credit
+across those charges oldest-first (`PaymentService::applyCredits`):
+
+- Each payment's `us_payments.credit_applied` is raised by the amount covered,
+  reducing what the student owes (`Payment::netDue()`).
+- A payment **fully** covered by credit is marked **paid-by-credit** (status
+  `paid`, registration confirmed) so it drops out of the admin confirmation queue.
+- A payment **partially** covered stays `pending` at its reduced net due, shown in
+  the admin Payments queue and on the notice.
+- The credit ledger is drawn down by the total applied
+  (`CreditRepository::consume`, FIFO), marking each spent credit `consumed`.
+
+The consolidated notice email (`Payment\PaymentDueMailer`) lists each charge at
+its full amount, then an **"Account credit applied: -X"** line and the reduced
+**Total due**. When the balance is zero the notice still goes out (so the student
+knows their credit covered it) but carries no e-transfer destination or reference.
+
+## Admin visibility
+The studio admin sees a student's credit on their **student detail** page (gated by
+`manage_billing`, like the payment history). An **Account credit** section shows the
+available balance and a table of every credit — date, reason, original amount,
+remaining, and status (`available` / `consumed`). Built by
+`Auth\StudentHistory::creditBalance` / `::credits`.
+
+## Data model — `{prefix}us_credits`
+
+| Column              | Type            | Notes                                             |
+|---------------------|-----------------|---------------------------------------------------|
+| `id`                | BIGINT UNSIGNED | Primary key                                       |
+| `student_id`        | BIGINT UNSIGNED | WordPress user ID                                 |
+| `amount`            | DECIMAL(10,2)   | Original credit amount                            |
+| `remaining`         | DECIMAL(10,2)   | Unused balance                                    |
+| `currency`          | VARCHAR(3)      | ISO 4217                                          |
+| `source_payment_id` | BIGINT UNSIGNED | Payment that paid for the cancelled lesson        |
+| `source_lesson_id`  | BIGINT UNSIGNED | The cancelled lesson (dedup key)                  |
+| `reason`            | VARCHAR(191)    | Human-readable note                               |
+| `status`            | VARCHAR(20)     | `available` / `consumed`                          |
+| `created_at`        | DATETIME        | Insertion time                                    |
+| `updated_at`        | DATETIME        | Last draw-down; NULL until first consumed         |
+
+A new column on `{prefix}us_payments`:
+
+| Column           | Type          | Notes                                                     |
+|------------------|---------------|-----------------------------------------------------------|
+| `credit_applied` | DECIMAL(10,2) | Account credit applied to this payment; `netDue = total − credit_applied` |
+
+> **Schema change:** `us_credits` and `us_payments.credit_applied` ship as part of
+> the (as-yet-unreleased) **1.2.0** — the same release as scheduled billing — so
+> `Installer`/`dbDelta` create them when a pre-1.2.0 site upgrades. If you are on a
+> 1.2.0 *dev* build that predates this feature, the stored `us_schedular_version`
+> already matches `USC_VERSION`, so `Plugin::boot()` will not re-run the installer;
+> reactivate the plugin (or bump the version) to pick the new table/column up.
+
+## Reporting caveat
+Credits never touch past revenue and a credit-covered future charge is still
+marked `paid`, so `PaymentReport` (which sums `status = paid`) counts the original
+paid lesson and the later credit-covered lesson as gross revenue. This mirrors the
+design choice to leave the original payment intact rather than represent a partial
+refund of a shared payment.
+
+## Implementation
+- Model: `Unsupervised\Schedular\Payment\Credit`
+- Repository: `Unsupervised\Schedular\Payment\CreditRepository`
+- Issue on cancel: `PaymentService::creditForCancelledLesson`
+  (called from `Booking\BookingEndpoint::cancel` and `::updateStatus`)
+- Apply at billing: `PaymentService::applyCredits`, driven by
+  `Payment\ScheduledBillingRunner::sendNotices`
+- Net due: `Payment::netDue()`, `PaymentRepository::addCreditApplied`
+- Lesson counts: `Booking\BookingRepository::countByPaymentId` / `countBySeries`
+- Admin view: `Auth\StudentHistory::creditBalance` / `::credits`, rendered in
+  `templates/admin/student-detail.php`
+
+## Tests
+- `tests/Unit/Payment/CreditRepositoryTest.php`
+- `tests/Unit/Payment/PaymentServiceTest.php` (`creditForCancelledLesson`, `applyCredits`)
+- `tests/Unit/Payment/ScheduledBillingRunnerTest.php` (credit applied to a run)
+- `tests/Unit/Payment/PaymentDueMailerTest.php` (credit line + reduced total)
+- `tests/Unit/Payment/PaymentTest.php` (`netDue`)
+- `tests/Unit/Booking/BookingEndpointTest.php` (credit issued on cancel)
+- `tests/Unit/Auth/StudentHistoryTest.php` (`creditBalance`, `credits`)