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/settings.php

@@ -0,0 +1,58 @@
+<?php
+declare(strict_types=1);
+
+if (! defined('ABSPATH')) {
+    exit;
+}
+
+/**
+ * @var string $publishableKey
+ * @var string $secretKey
+ * @var string $mode
+ * @var string $currency
+ * @var bool   $stripeConfigured
+ */
+?>
+<div class="wrap">
+    <h1><?php esc_html_e('Studio Settings', 'unsupervised-schedular'); ?></h1>
+
+    <div class="notice notice-info inline">
+        <p>
+            <?php if ($stripeConfigured) : ?>
+                <?php esc_html_e('Stripe is configured — new registrations default to credit-card billing.', 'unsupervised-schedular'); ?>
+            <?php else : ?>
+                <?php esc_html_e('Stripe is not configured — new registrations default to e-transfer, which a studio admin marks paid on receipt. Add your Stripe keys below to enable card billing.', 'unsupervised-schedular'); ?>
+            <?php endif; ?>
+        </p>
+    </div>
+
+    <h2><?php esc_html_e('Stripe', 'unsupervised-schedular'); ?></h2>
+    <form method="post">
+        <?php wp_nonce_field('usc_settings_action'); ?>
+        <input type="hidden" name="usc_action" value="save">
+        <table class="form-table">
+            <tr>
+                <th><label for="publishable_key"><?php esc_html_e('Publishable key', 'unsupervised-schedular'); ?></label></th>
+                <td><input type="text" name="publishable_key" id="publishable_key" class="regular-text" value="<?php echo esc_attr($publishableKey); ?>" autocomplete="off"></td>
+            </tr>
+            <tr>
+                <th><label for="secret_key"><?php esc_html_e('Secret key', 'unsupervised-schedular'); ?></label></th>
+                <td><input type="password" name="secret_key" id="secret_key" class="regular-text" value="<?php echo esc_attr($secretKey); ?>" autocomplete="off"></td>
+            </tr>
+            <tr>
+                <th><label for="mode"><?php esc_html_e('Mode', 'unsupervised-schedular'); ?></label></th>
+                <td>
+                    <select name="mode" id="mode">
+                        <option value="test" <?php selected($mode, 'test'); ?>><?php esc_html_e('Test', 'unsupervised-schedular'); ?></option>
+                        <option value="live" <?php selected($mode, 'live'); ?>><?php esc_html_e('Live', 'unsupervised-schedular'); ?></option>
+                    </select>
+                </td>
+            </tr>
+            <tr>
+                <th><label for="currency"><?php esc_html_e('Default currency', 'unsupervised-schedular'); ?></label></th>
+                <td><input type="text" name="currency" id="currency" class="small-text" maxlength="3" value="<?php echo esc_attr($currency); ?>"></td>
+            </tr>
+        </table>
+        <?php submit_button(esc_html__('Save Settings', 'unsupervised-schedular')); ?>
+    </form>
+</div>