Add multi-use group invite links with expiry and auto-approval on email confirmation

A studio admin can generate a shareable group invite link (e.g. for a
newsletter) from the Invites page, choosing a required expiry date. Anyone
with the link may register while it is valid, in any registration mode: the
form collects their own email, they must confirm it via the usual hashed
token, and confirming approves the account immediately — group signups never
enter the Pending Students queue.

- us_invites grows kind (personal/group) and expires_at; an explicit expiry
  wins over the personal 14-day window. Group links stay pending (multi-use)
  until revoked or expired.
- RegistrationPage: group signups create the account pending with the
  us_auto_approve marker and send the confirmation email; no auto-login.
- EmailConfirmationHandler: auto-approve accounts are approved on
  confirmation, emailed the approved notice, and redirected to a new
  us_confirmed=ready notice with a sign-in link.

Closes #77

Co-Authored-By: Claude Fable 5 <[email protected]>

templates/admin/invites.php

@@ -66,6 +66,23 @@ if (! defined('ABSPATH')) {
         <?php submit_button(esc_html__('Generate Invitation Link', 'unsupervised-schedular')); ?>
     </form>
 
+    <h2><?php esc_html_e('Group Invite Link', 'unsupervised-schedular'); ?></h2>
+    <p class="description"><?php esc_html_e('Generate a shareable link (e.g. for a newsletter). Anyone with the link can register until it expires: they enter their own email and must confirm it, but no admin approval is needed afterwards.', 'unsupervised-schedular'); ?></p>
+    <form method="post">
+        <?php wp_nonce_field('usc_invite_action'); ?>
+        <input type="hidden" name="usc_action" value="group_invite">
+        <table class="form-table">
+            <tr>
+                <th><label for="expires_at"><?php esc_html_e('Expires on', 'unsupervised-schedular'); ?></label></th>
+                <td>
+                    <input type="date" name="expires_at" id="expires_at" required min="<?php echo esc_attr((string) current_time('Y-m-d')); ?>">
+                    <p class="description"><?php esc_html_e('The link stops working at the end of this day.', 'unsupervised-schedular'); ?></p>
+                </td>
+            </tr>
+        </table>
+        <?php submit_button(esc_html__('Generate Group Link', 'unsupervised-schedular')); ?>
+    </form>
+
     <h2><?php esc_html_e('Pending Invites', 'unsupervised-schedular'); ?></h2>
 
     <?php if (empty($pendingInvites)) : ?>
@@ -74,8 +91,9 @@ if (! defined('ABSPATH')) {
         <table class="wp-list-table widefat fixed striped">
             <thead>
                 <tr>
-                    <th><?php esc_html_e('Email', 'unsupervised-schedular'); ?></th>
-                    <th><?php esc_html_e('Invited', 'unsupervised-schedular'); ?></th>
+                    <th><?php esc_html_e('Invite', 'unsupervised-schedular'); ?></th>
+                    <th><?php esc_html_e('Created', 'unsupervised-schedular'); ?></th>
+                    <th><?php esc_html_e('Expires', 'unsupervised-schedular'); ?></th>
                     <th><?php esc_html_e('Actions', 'unsupervised-schedular'); ?></th>
                 </tr>
             </thead>
@@ -84,7 +102,7 @@ if (! defined('ABSPATH')) {
                 <?php foreach ($pendingInvites as $invite) : ?>
                     <tr>
                         <td>
-                            <?php echo esc_html($invite->email); ?>
+                            <?php echo $invite->isGroup() ? esc_html__('Group link', 'unsupervised-schedular') : esc_html($invite->email); ?>
                             <?php if ($invite->isExpired($now)) : ?>
                                 <span class="us-invite-expired" style="color:#b32d2e;">— <?php esc_html_e('expired', 'unsupervised-schedular'); ?></span>
                             <?php endif; ?>
@@ -92,6 +110,9 @@ if (! defined('ABSPATH')) {
                         <td>
                             <?php echo esc_html((string) $invite->createdAt); ?>
                         </td>
+                        <td>
+                            <?php echo esc_html($invite->expiresAt !== null ? (string) mysql2date('M j, Y', $invite->expiresAt) : '—'); ?>
+                        </td>
                         <td>
                             <form method="post" style="display:inline;">
                                 <?php wp_nonce_field('usc_invite_action'); ?>