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/AdminMenu.php

@@ -15,6 +15,11 @@ use Unsupervised\Schedular\GroupClass\EnrollmentRepository;
 use Unsupervised\Schedular\GroupClass\GroupClassController;
 use Unsupervised\Schedular\Offering\OfferingController;
 use Unsupervised\Schedular\Offering\OfferingRepository;
+use Unsupervised\Schedular\Payment\BillingMethodResolver;
+use Unsupervised\Schedular\Payment\PaymentController;
+use Unsupervised\Schedular\Payment\PaymentRepository;
+use Unsupervised\Schedular\Payment\PaymentService;
+use Unsupervised\Schedular\Payment\StudioSettings;
 use Unsupervised\Schedular\Policy\PolicyController;
 use Unsupervised\Schedular\Policy\PolicyRepository;
 use Unsupervised\Schedular\Policy\PolicyService;
@@ -32,8 +37,10 @@ class AdminMenu {
 	private RegistrationController $registrationController;
 	private GroupClassController $groupClassController;
 	private StudentController $studentController;
+	private StudioSettings $settings;
+	private PaymentController $paymentController;
 
-	public function __construct( AvailabilityRepository $availability, BookingRepository $bookings, OfferingRepository $offerings, QuestionRepository $questions, PolicyRepository $policies, PolicyVersionRepository $policyVersions, PolicyService $policyService, InviteRepository $invites, EnrollmentRepository $enrollments ) {
+	public function __construct( AvailabilityRepository $availability, BookingRepository $bookings, OfferingRepository $offerings, QuestionRepository $questions, PolicyRepository $policies, PolicyVersionRepository $policyVersions, PolicyService $policyService, InviteRepository $invites, EnrollmentRepository $enrollments, StudioSettings $settings, PaymentRepository $payments, PaymentService $paymentService, BillingMethodResolver $resolver ) {
 		$this->availabilityController = new AvailabilityController( $availability, $offerings );
 		$this->lessonController       = new LessonController( $bookings );
 		$this->offeringController     = new OfferingController( $offerings );
@@ -41,7 +48,9 @@ class AdminMenu {
 		$this->policyController       = new PolicyController( $policies, $policyVersions, $policyService );
 		$this->registrationController = new RegistrationController( $invites );
 		$this->groupClassController   = new GroupClassController( $enrollments, $offerings );
-		$this->studentController      = new StudentController( $bookings, $availability, $offerings, $enrollments );
+		$this->studentController      = new StudentController( $bookings, $availability, $offerings, $enrollments, $resolver );
+		$this->settings               = $settings;
+		$this->paymentController      = new PaymentController( $payments, $paymentService );
 	}
 
 	public function register(): void {
@@ -136,6 +145,28 @@ class AdminMenu {
 			37
 		);
 
+		// Studio admin: confirm pending (e-transfer) payments.
+		add_menu_page(
+			__( 'Payments', 'unsupervised-schedular' ),
+			__( 'Payments', 'unsupervised-schedular' ),
+			RoleManager::CAP_MANAGE_BILLING,
+			'us-payments',
+			[ $this->paymentController, 'renderPage' ],
+			'dashicons-money-alt',
+			38
+		);
+
+		// Studio admin: Stripe credentials and billing settings.
+		add_menu_page(
+			__( 'Studio Settings', 'unsupervised-schedular' ),
+			__( 'Studio Settings', 'unsupervised-schedular' ),
+			RoleManager::CAP_MANAGE_BILLING,
+			'us-settings',
+			[ $this->settings, 'renderPage' ],
+			'dashicons-admin-settings',
+			39
+		);
+
 		// Instructor: view their upcoming lessons.
 		add_menu_page(
 			__( 'My Lessons', 'unsupervised-schedular' ),