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>

README.md

@@ -35,7 +35,7 @@ model, REST API, classes, and tests. For contributor/architecture guidance see
 | Lesson booking (offering → questions → policies) | [lesson-booking.md](docs/features/lesson-booking.md) | ✅ Implemented |
 | Group classes (capacity-enforced enrolment) | [group-classes.md](docs/features/group-classes.md) | ✅ Implemented |
 | Student administration (studio-admin view) | [student-administration.md](docs/features/student-administration.md) | ✅ Implemented |
-| Payments (Stripe, e-transfer/comp, receipts) | [payments.md](docs/features/payments.md) | 🟡 Planned |
+| Payments (e-transfer/comp + receipts; Stripe card charge pending) | [payments.md](docs/features/payments.md) | 🟡 Partial |
 | Payment reporting (monthly per-instructor + CSV) | [payment-reporting.md](docs/features/payment-reporting.md) | 🟡 Planned |
 
 > Payments are deliberately deferred to the end: booking and enrolment ship with a