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>

tests/Unit/Payment/PaymentTest.php

@@ -0,0 +1,63 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Tests\Unit\Payment;
+
+use Unsupervised\Schedular\Payment\Payment;
+use Unsupervised\Schedular\Tests\Unit\TestCase;
+
+class PaymentTest extends TestCase
+{
+    public function testDefaults(): void
+    {
+        $payment = new Payment(5, 3, Payment::REG_LESSON, 12, 35.00);
+
+        self::assertSame('CAD', $payment->currency);
+        self::assertSame(Payment::METHOD_ETRANSFER, $payment->method);
+        self::assertSame(Payment::STATUS_PENDING, $payment->status);
+        self::assertFalse($payment->isPaid());
+        self::assertNull($payment->id);
+    }
+
+    public function testConstants(): void
+    {
+        self::assertContains(Payment::METHOD_CARD, Payment::VALID_METHODS);
+        self::assertContains(Payment::METHOD_ETRANSFER, Payment::VALID_METHODS);
+        self::assertContains(Payment::METHOD_COMP, Payment::VALID_METHODS);
+        self::assertContains(Payment::STATUS_PAID, Payment::VALID_STATUSES);
+    }
+
+    public function testFromRowMapsCorrectly(): void
+    {
+        $payment = Payment::fromRow((object) [
+            'id'                       => '7',
+            'student_id'               => '5',
+            'instructor_id'            => '3',
+            'registration_type'        => Payment::REG_ENROLLMENT,
+            'registration_id'          => '12',
+            'amount'                   => '120.00',
+            'currency'                 => 'CAD',
+            'method'                   => Payment::METHOD_COMP,
+            'status'                   => Payment::STATUS_PAID,
+            'stripe_payment_intent_id' => null,
+            'receipt_number'           => 'USC-7',
+            'receipt_sent_at'          => null,
+            'paid_at'                  => '2026-06-08 10:00:00',
+        ]);
+
+        self::assertSame(7, $payment->id);
+        self::assertSame(120.00, $payment->amount);
+        self::assertSame(Payment::METHOD_COMP, $payment->method);
+        self::assertTrue($payment->isPaid());
+        self::assertSame('USC-7', $payment->receiptNumber);
+    }
+
+    public function testToArrayContainsExpectedKeys(): void
+    {
+        $arr = (new Payment(5, 3, Payment::REG_LESSON, 12, 35.00, id: 7))->toArray();
+
+        foreach (['id', 'student_id', 'amount', 'currency', 'method', 'status', 'receipt_number'] as $key) {
+            self::assertArrayHasKey($key, $arr);
+        }
+    }
+}