Add HST/tax support and payment reporting with HST aggregation

Studio Settings gains a default HST rate; the rate is frozen onto each
payment at booking and computed against the pre-tax subtotal, with the
total billed as subtotal + tax. The rate is overridable per booking on
My Lessons while unpaid (recomputing the tax amount), comped
registrations are never taxed, and receipts break out subtotal/HST/total.

Builds the payments report (roadmap #8) from us_payments: a monthly
per-instructor view with subtotal, HST collected, and grand-total
aggregation, plus a nonce-protected CSV export via admin-post. Studio
admins see all instructors and can filter; instructors are scoped to
their own rows. The Payment Report menu is gated on export_payments so
instructors (who lack manage_billing) can reach it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/admin/lessons.php

@@ -5,7 +5,7 @@ if (! defined('ABSPATH')) {
     exit;
 }
 
-/** @var list<array{student: string, instructor: string, slot_id: int, status: string, notes: string, payment_id: int, etransfer_email: string, etransfer_editable: bool}> $rows */
+/** @var list<array{student: string, instructor: string, slot_id: int, status: string, notes: string, payment_id: int, currency: string, amount: float, tax_rate: float, tax_amount: float, total: float, etransfer_email: string, etransfer_editable: bool, tax_editable: bool}> $rows */
 ?>
 <div class="wrap">
     <h1><?php esc_html_e('Lessons', 'unsupervised-schedular'); ?></h1>
@@ -20,6 +20,8 @@ if (! defined('ABSPATH')) {
                     <th><?php esc_html_e('Instructor', 'unsupervised-schedular'); ?></th>
                     <th><?php esc_html_e('Slot ID', 'unsupervised-schedular'); ?></th>
                     <th><?php esc_html_e('Status', 'unsupervised-schedular'); ?></th>
+                    <th><?php esc_html_e('HST', 'unsupervised-schedular'); ?></th>
+                    <th><?php esc_html_e('Total', 'unsupervised-schedular'); ?></th>
                     <th><?php esc_html_e('E-transfer email', 'unsupervised-schedular'); ?></th>
                     <th><?php esc_html_e('Notes', 'unsupervised-schedular'); ?></th>
                 </tr>
@@ -31,6 +33,21 @@ if (! defined('ABSPATH')) {
                         <td><?php echo esc_html($row['instructor']); ?></td>
                         <td><?php echo esc_html((string) $row['slot_id']); ?></td>
                         <td><?php echo esc_html($row['status']); ?></td>
+                        <td>
+                            <?php if ($row['tax_editable']) : ?>
+                                <form method="post" style="display:flex; gap:4px; align-items:center;">
+                                    <?php wp_nonce_field('usc_lesson_action'); ?>
+                                    <input type="hidden" name="usc_action" value="set_tax">
+                                    <input type="hidden" name="payment_id" value="<?php echo esc_attr((string) $row['payment_id']); ?>">
+                                    <input type="number" name="tax_rate" min="0" max="100" step="0.01" style="width:5em;" value="<?php echo esc_attr(number_format($row['tax_rate'], 2)); ?>">
+                                    <span>%</span>
+                                    <button type="submit" class="button button-small"><?php esc_html_e('Save', 'unsupervised-schedular'); ?></button>
+                                </form>
+                            <?php else : ?>
+                                <?php echo esc_html(number_format($row['tax_rate'], 2) . '% (' . $row['currency'] . ' ' . number_format($row['tax_amount'], 2) . ')'); ?>
+                            <?php endif; ?>
+                        </td>
+                        <td><?php echo $row['payment_id'] > 0 ? esc_html($row['currency'] . ' ' . number_format($row['total'], 2)) : esc_html('—'); ?></td>
                         <td>
                             <?php if ($row['etransfer_editable']) : ?>
                                 <form method="post" style="display:flex; gap:4px;">