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>

templates/admin/student-detail.php

@@ -11,6 +11,9 @@ if (! defined('ABSPATH')) {
  * @var list<array{start_dt: string, end_dt: string, offering: string, instructor: string, status: string}> $past
  * @var list<array{offering: string, status: string}> $enrolments
  * @var string $backUrl
+ * @var bool   $canBilling
+ * @var string $billingOverride
+ * @var string $billingDefault
  */
 
 $renderLessons = static function (array $rows): void {
@@ -54,6 +57,26 @@ $renderLessons = static function (array $rows): void {
         <tr><th><?php esc_html_e('Registered', 'unsupervised-schedular'); ?></th><td><?php echo esc_html($student->user_registered); ?></td></tr>
     </table>
 
+    <?php if ($canBilling) : ?>
+        <h2><?php esc_html_e('Billing method', 'unsupervised-schedular'); ?></h2>
+        <form method="post">
+            <?php wp_nonce_field('usc_student_billing'); ?>
+            <input type="hidden" name="usc_action" value="set_billing">
+            <select name="payment_method">
+                <option value="">
+                    <?php
+                    /* translators: %s: the studio default billing method */
+                    echo esc_html(sprintf(__('Studio default (%s)', 'unsupervised-schedular'), $billingDefault));
+                    ?>
+                </option>
+                <option value="card" <?php selected($billingOverride, 'card'); ?>><?php esc_html_e('Credit card', 'unsupervised-schedular'); ?></option>
+                <option value="etransfer" <?php selected($billingOverride, 'etransfer'); ?>><?php esc_html_e('E-transfer', 'unsupervised-schedular'); ?></option>
+                <option value="comp" <?php selected($billingOverride, 'comp'); ?>><?php esc_html_e('Comp (no charge)', 'unsupervised-schedular'); ?></option>
+            </select>
+            <?php submit_button(esc_html__('Save', 'unsupervised-schedular'), 'secondary', 'submit', false); ?>
+        </form>
+    <?php endif; ?>
+
     <h2><?php esc_html_e('Upcoming lessons', 'unsupervised-schedular'); ?></h2>
     <?php $renderLessons($upcoming); ?>