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/BillingMethodResolverTest.php

@@ -0,0 +1,58 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Tests\Unit\Payment;
+
+use Brain\Monkey\Functions;
+use Mockery;
+use Unsupervised\Schedular\Payment\BillingMethodResolver;
+use Unsupervised\Schedular\Payment\Payment;
+use Unsupervised\Schedular\Payment\StudioSettings;
+use Unsupervised\Schedular\Tests\Unit\TestCase;
+
+class BillingMethodResolverTest extends TestCase
+{
+    public function testPerStudentOverrideWins(): void
+    {
+        Functions\when('get_user_meta')->justReturn(Payment::METHOD_COMP);
+
+        $settings = Mockery::mock(StudioSettings::class);
+        $resolver = new BillingMethodResolver($settings);
+
+        self::assertSame(Payment::METHOD_COMP, $resolver->resolve(5));
+    }
+
+    public function testDefaultsToCardWhenStripeConfigured(): void
+    {
+        Functions\when('get_user_meta')->justReturn('');
+
+        $settings = Mockery::mock(StudioSettings::class);
+        $settings->shouldReceive('isStripeConfigured')->andReturn(true);
+        $resolver = new BillingMethodResolver($settings);
+
+        self::assertSame(Payment::METHOD_CARD, $resolver->resolve(5));
+    }
+
+    public function testDefaultsToEtransferWhenStripeNotConfigured(): void
+    {
+        Functions\when('get_user_meta')->justReturn('');
+
+        $settings = Mockery::mock(StudioSettings::class);
+        $settings->shouldReceive('isStripeConfigured')->andReturn(false);
+        $resolver = new BillingMethodResolver($settings);
+
+        self::assertSame(Payment::METHOD_ETRANSFER, $resolver->resolve(5));
+        self::assertSame(Payment::METHOD_ETRANSFER, $resolver->defaultMethod());
+    }
+
+    public function testInvalidOverrideFallsBackToDefault(): void
+    {
+        Functions\when('get_user_meta')->justReturn('bogus');
+
+        $settings = Mockery::mock(StudioSettings::class);
+        $settings->shouldReceive('isStripeConfigured')->andReturn(true);
+        $resolver = new BillingMethodResolver($settings);
+
+        self::assertSame(Payment::METHOD_CARD, $resolver->resolve(5));
+    }
+}