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/RestRegistrar.php

@@ -11,6 +11,8 @@ use Unsupervised\Schedular\GroupClass\EnrollmentEndpoint;
 use Unsupervised\Schedular\GroupClass\EnrollmentRepository;
 use Unsupervised\Schedular\Offering\OfferingEndpoint;
 use Unsupervised\Schedular\Offering\OfferingRepository;
+use Unsupervised\Schedular\Payment\PaymentEndpoint;
+use Unsupervised\Schedular\Payment\PaymentService;
 use Unsupervised\Schedular\Policy\PolicyEndpoint;
 use Unsupervised\Schedular\Policy\PolicyRepository;
 use Unsupervised\Schedular\Policy\PolicyService;
@@ -29,14 +31,16 @@ class RestRegistrar {
 	private QuestionEndpoint $questionEndpoint;
 	private PolicyEndpoint $policyEndpoint;
 	private EnrollmentEndpoint $enrollmentEndpoint;
+	private PaymentEndpoint $paymentEndpoint;
 
-	public function __construct( AvailabilityRepository $availability, BookingRepository $bookings, OfferingRepository $offerings, QuestionRepository $questions, PolicyRepository $policies, PolicyVersionRepository $policyVersions, PolicyService $policyService, RegistrationGate $gate, EnrollmentRepository $enrollments ) {
+	public function __construct( AvailabilityRepository $availability, BookingRepository $bookings, OfferingRepository $offerings, QuestionRepository $questions, PolicyRepository $policies, PolicyVersionRepository $policyVersions, PolicyService $policyService, RegistrationGate $gate, EnrollmentRepository $enrollments, PaymentService $paymentService ) {
 		$this->availabilityEndpoint = new AvailabilityEndpoint( $availability );
-		$this->bookingEndpoint      = new BookingEndpoint( $availability, $bookings, $offerings, $gate );
+		$this->bookingEndpoint      = new BookingEndpoint( $availability, $bookings, $offerings, $gate, $paymentService );
 		$this->offeringEndpoint     = new OfferingEndpoint( $offerings );
 		$this->questionEndpoint     = new QuestionEndpoint( $questions, $offerings );
 		$this->policyEndpoint       = new PolicyEndpoint( $policies, $policyVersions, $policyService );
-		$this->enrollmentEndpoint   = new EnrollmentEndpoint( $enrollments, $offerings, $gate );
+		$this->enrollmentEndpoint   = new EnrollmentEndpoint( $enrollments, $offerings, $gate, $paymentService );
+		$this->paymentEndpoint      = new PaymentEndpoint( $paymentService );
 	}
 
 	public function register(): void {
@@ -50,5 +54,6 @@ class RestRegistrar {
 		$this->questionEndpoint->registerRoutes( self::NAMESPACE );
 		$this->policyEndpoint->registerRoutes( self::NAMESPACE );
 		$this->enrollmentEndpoint->registerRoutes( self::NAMESPACE );
+		$this->paymentEndpoint->registerRoutes( self::NAMESPACE );
 	}
 }