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>

src/Booking/BookingEndpoint.php

@@ -6,6 +6,8 @@ namespace Unsupervised\Schedular\Booking;
 use Unsupervised\Schedular\Availability\AvailabilityRepository;
 use Unsupervised\Schedular\Auth\RoleManager;
 use Unsupervised\Schedular\Offering\OfferingRepository;
+use Unsupervised\Schedular\Payment\Payment;
+use Unsupervised\Schedular\Payment\PaymentService;
 use Unsupervised\Schedular\Policy\PolicyAcceptance;
 use Unsupervised\Schedular\Registration\RegistrationGate;
 
@@ -16,6 +18,7 @@ class BookingEndpoint {
 		private BookingRepository $bookings,
 		private OfferingRepository $offerings,
 		private RegistrationGate $gate,
+		private PaymentService $payments,
 	) {}
 
 	public function registerRoutes( string $route_namespace ): void {
@@ -109,7 +112,8 @@ class BookingEndpoint {
 		if ( 0 === $offeringId ) {
 			$offeringId = (int) ( $slot->offeringId ?? 0 );
 		}
-		if ( $offeringId > 0 && null === $this->offerings->findById( $offeringId ) ) {
+		$offering = $offeringId > 0 ? $this->offerings->findById( $offeringId ) : null;
+		if ( $offeringId > 0 && null === $offering ) {
 			return new \WP_Error( 'invalid_offering', __( 'Offering not found.', 'unsupervised-schedular' ), [ 'status' => 400 ] );
 		}
 
@@ -152,6 +156,10 @@ class BookingEndpoint {
 
 		$this->gate->record( PolicyAcceptance::REG_LESSON, $anchorId, $studentId, $offeringId, $answers, $acceptedVersionIds, $this->clientIp() );
 
+		if ( null !== $offering && $offering->price > 0.0 ) {
+			$this->payments->createForRegistration( Payment::REG_LESSON, $anchorId, $studentId, $slot->instructorId, $offering->price, $offering->currency );
+		}
+
 		return new \WP_REST_Response(
 			[
 				'ids'    => $ids,