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/settings.php

@@ -11,6 +11,7 @@ if (! defined('ABSPATH')) {
  * @var string $mode
  * @var string $currency
  * @var string $etransferEmail
+ * @var float  $hstRate
  * @var bool   $stripeConfigured
  */
 ?>
@@ -53,6 +54,13 @@ if (! defined('ABSPATH')) {
                 <th><label for="currency"><?php esc_html_e('Default currency', 'unsupervised-schedular'); ?></label></th>
                 <td><input type="text" name="currency" id="currency" class="small-text" maxlength="3" value="<?php echo esc_attr($currency); ?>"></td>
             </tr>
+            <tr>
+                <th><label for="hst_rate"><?php esc_html_e('Default HST rate (%)', 'unsupervised-schedular'); ?></label></th>
+                <td>
+                    <input type="number" name="hst_rate" id="hst_rate" class="small-text" min="0" max="100" step="0.01" value="<?php echo esc_attr(number_format($hstRate, 2)); ?>">
+                    <p class="description"><?php esc_html_e('Added to the subtotal when billing. 0 means no tax. Overridable per booking on My Lessons.', 'unsupervised-schedular'); ?></p>
+                </td>
+            </tr>
         </table>
 
         <h2><?php esc_html_e('E-transfer', 'unsupervised-schedular'); ?></h2>