Add payments foundation (e-transfer/comp, Stripe config, receipts)

Implements the payments foundation for #7. Without Stripe credentials
everything works on e-transfer (pending payment confirmed by a studio
admin); when Stripe keys are configured the default flips to credit card.
Per-student override (card/etransfer/comp) is set on the student detail.

- Schema: us_payments (amount DECIMAL dollars, method, status, receipt,
  stripe intent id).
- src/Payment/: Payment VO, PaymentRepository, StudioSettings (Stripe
  options + isStripeConfigured + settings page), BillingMethodResolver
  (per-student override; default card if configured else etransfer),
  ReceiptMailer, PaymentService (create at registration, link payment_id,
  comp->paid+confirm, markPaid->confirm+receipt), PaymentController
  (e-transfer confirmation queue), PaymentEndpoint (PATCH /payments/{id}).
- Booking + enrolment create the payment from the offering price; comp
  auto-confirms the lesson; setPaymentId on both repositories.
- Admin: Studio Settings + Payments menus (manage_billing); per-student
  billing method on the student detail page.
- Docs: payments.md + README updated.

Deferred to a follow-up: the live Stripe card charge (PaymentIntent +
Stripe.js Elements + webhook + stripe/stripe-php). Until then a card
payment is created pending and confirmed like an e-transfer.

Tests: tests/Unit/Payment/ (VO, repository, resolver, service, mailer).
composer test (147), cs, and PHPStan level 6 all pass.

Refs #7

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/features/payments.md

@@ -1,7 +1,22 @@
 # Feature: Payments
 
 ## Overview
-Payment is taken at registration. The default rail is a credit card charged through Stripe, but the studio admin can set any student to pay by e-transfer (recorded, marked paid manually) or to be comped (no charge). Single bookings are charged once; weekly reservations and group classes are charged the full term upfront. A numbered receipt is emailed automatically when a payment is marked paid.
+Payment is taken at registration. When Stripe is **not** configured the platform
+falls back to **e-transfer** — a pending payment a studio admin marks received —
+so everything works without any credentials. When Stripe **is** configured the
+default rail becomes the **credit card**. The studio admin can override any
+student's method (card / e-transfer / comp). Single bookings are charged once;
+weekly reservations and group classes are charged the full term upfront. A
+numbered receipt is emailed automatically when a payment is marked paid.
+
+> **Implemented:** the payment ledger, studio settings, method resolution
+> (e-transfer default with no Stripe; card default when configured; per-student
+> override), the e-transfer/comp flow with admin confirmation + receipts, and
+> integration into booking/enrolment (a registration's `payment_id` is linked;
+> comp auto-confirms; e-transfer stays pending until confirmed).
+> **Deferred to a follow-up:** the live Stripe card charge (PaymentIntent +
+> Stripe.js Elements + webhook + `stripe/stripe-php`). Until then a `card`
+> payment is created `pending` and can be confirmed like an e-transfer.
 
 ## Stripe Configuration
 Stripe credentials live in WordPress options, managed on the **Studio Settings**
@@ -16,7 +31,9 @@ page (`manage_billing`, studio admin only):
 
 ## Per-Student Billing Method
 Each student's billing method is stored in user meta `us_payment_method`, set by the
-studio admin (default `card`):
+studio admin (`Students → student detail → Billing method`). When unset, the studio
+default applies — `card` if Stripe is configured, otherwise `etransfer`
+(`BillingMethodResolver`):
 
 | Method     | Behaviour                                                              |
 |------------|-----------------------------------------------------------------------|
@@ -33,7 +50,7 @@ studio admin (default `card`):
 | `instructor_id`            | BIGINT UNSIGNED  | WordPress user ID (denormalised for reporting)        |
 | `registration_type`        | VARCHAR(20)      | `lesson` or `enrollment`                              |
 | `registration_id`          | BIGINT UNSIGNED  | FK → `us_lessons.id` or `us_group_enrollments.id`     |
-| `amount_cents`             | INT UNSIGNED     | Charged amount in the smallest currency unit          |
+| `amount`                   | DECIMAL(10,2)    | Charged amount in dollars (matches the offering price) |
 | `currency`                 | VARCHAR(3)       | ISO 4217, e.g. `CAD`                                  |
 | `method`                   | VARCHAR(20)      | `card` / `etransfer` / `comp`                         |
 | `status`                   | VARCHAR(20)      | `pending` / `paid` / `failed` / `refunded`            |