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/PaymentRepositoryTest.php

@@ -0,0 +1,107 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Tests\Unit\Payment;
+
+use Brain\Monkey\Functions;
+use Mockery;
+use Unsupervised\Schedular\Payment\Payment;
+use Unsupervised\Schedular\Payment\PaymentRepository;
+use Unsupervised\Schedular\Tests\Unit\TestCase;
+
+class PaymentRepositoryTest extends TestCase
+{
+    private \wpdb $db;
+    private PaymentRepository $repo;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        $this->db         = Mockery::mock(\wpdb::class);
+        $this->db->prefix = 'wp_';
+        $this->repo       = new PaymentRepository($this->db);
+    }
+
+    public function testInsertReturnsId(): void
+    {
+        Functions\expect('current_time')->with('mysql')->andReturn('2026-06-08 12:00:00');
+
+        $this->db->shouldReceive('insert')
+            ->once()
+            ->with(
+                'wp_us_payments',
+                Mockery::on(static function (array $d): bool {
+                    return $d['student_id'] === 5
+                        && $d['amount'] === 35.00
+                        && $d['method'] === Payment::METHOD_ETRANSFER
+                        && $d['status'] === Payment::STATUS_PENDING;
+                }),
+                Mockery::type('array')
+            );
+        $this->db->insert_id = 50;
+
+        self::assertSame(50, $this->repo->insert(new Payment(5, 3, Payment::REG_LESSON, 12, 35.00)));
+    }
+
+    public function testMarkPaidUpdatesStatusAndReceipt(): void
+    {
+        Functions\expect('current_time')->with('mysql')->andReturn('2026-06-08 12:00:00');
+
+        $this->db->shouldReceive('update')
+            ->once()
+            ->with(
+                'wp_us_payments',
+                Mockery::on(static fn (array $d): bool => $d['status'] === Payment::STATUS_PAID && $d['receipt_number'] === 'USC-50' && $d['paid_at'] === '2026-06-08 12:00:00'),
+                ['id' => 50],
+                ['%s', '%s', '%s'],
+                ['%d']
+            )
+            ->andReturn(1);
+
+        self::assertTrue($this->repo->markPaid(50, 'USC-50'));
+    }
+
+    public function testFindByRegistrationReturnsPayment(): void
+    {
+        $this->db->shouldReceive('prepare')
+            ->once()
+            ->with(Mockery::pattern('/registration_type = %s AND registration_id = %d/'), Payment::REG_LESSON, 12)
+            ->andReturn('SELECT ...');
+
+        $this->db->shouldReceive('get_row')->andReturn($this->row());
+
+        self::assertInstanceOf(Payment::class, $this->repo->findByRegistration(Payment::REG_LESSON, 12));
+    }
+
+    public function testFindPendingMapsRows(): void
+    {
+        $this->db->shouldReceive('prepare')
+            ->once()
+            ->with(Mockery::pattern('/status = %s/'), Payment::STATUS_PENDING)
+            ->andReturn('SELECT ...');
+
+        $this->db->shouldReceive('get_results')->andReturn([$this->row()]);
+
+        self::assertCount(1, $this->repo->findPending());
+    }
+
+    private function row(): object
+    {
+        return (object) [
+            'id'                       => '50',
+            'student_id'               => '5',
+            'instructor_id'            => '3',
+            'registration_type'        => Payment::REG_LESSON,
+            'registration_id'          => '12',
+            'amount'                   => '35.00',
+            'currency'                 => 'CAD',
+            'method'                   => Payment::METHOD_ETRANSFER,
+            'status'                   => Payment::STATUS_PENDING,
+            'stripe_payment_intent_id' => null,
+            'receipt_number'           => null,
+            'receipt_sent_at'          => null,
+            'paid_at'                  => null,
+        ];
+    }
+}