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/frontend/register-page.php

@@ -11,8 +11,8 @@ if (! defined('ABSPATH')) {
  * @var string $token Raw invite token from the request (only its hash is stored).
  * @var bool   $canRegister
  * @var bool   $open   Whether open (self-approval) registration is enabled.
- * @var string $successType '' | 'invite' (created + logged in) | 'confirm' (check email).
- * @var string $confirmResult '' | '1' (email confirmed) | 'expired'.
+ * @var string $successType '' | 'invite' (created + logged in) | 'confirm' (check email) | 'confirm_group' (check email; auto-approved on confirm).
+ * @var string $confirmResult '' | '1' (email confirmed, awaiting approval) | 'ready' (confirmed + auto-approved) | 'expired'.
  * @var string $loginUrl Where the post-confirmation sign-in link points.
  * @var string $error
  * @var list<array{policy: \Unsupervised\Schedular\Policy\Policy, version: \Unsupervised\Schedular\Policy\PolicyVersion}> $policyForms
@@ -23,6 +23,11 @@ if (! defined('ABSPATH')) {
         <p class="us-success"><?php esc_html_e('Your account has been created and you are now logged in.', 'unsupervised-schedular'); ?></p>
     <?php elseif ($successType === 'confirm') : ?>
         <p class="us-success"><?php esc_html_e('Your account has been created. Check your email for a link to confirm your address — once you do, a studio admin will review and approve your account.', 'unsupervised-schedular'); ?></p>
+    <?php elseif ($successType === 'confirm_group') : ?>
+        <p class="us-success"><?php esc_html_e('Your account has been created. Check your email for a link to confirm your address — once you do, your account is ready to use.', 'unsupervised-schedular'); ?></p>
+    <?php elseif ($confirmResult === 'ready') : ?>
+        <p class="us-success"><?php esc_html_e('Thanks — your email is confirmed and your account is ready to use.', 'unsupervised-schedular'); ?></p>
+        <p><a href="<?php echo esc_url($loginUrl); ?>"><?php esc_html_e('Sign in to your account', 'unsupervised-schedular'); ?></a></p>
     <?php elseif ($confirmResult === '1') : ?>
         <p class="us-success"><?php esc_html_e('Thanks — your email is confirmed. Your account is now awaiting studio approval; we will email you when it is ready.', 'unsupervised-schedular'); ?></p>
         <p><a href="<?php echo esc_url($loginUrl); ?>"><?php esc_html_e('Sign in to your account', 'unsupervised-schedular'); ?></a></p>
@@ -44,7 +49,7 @@ if (! defined('ABSPATH')) {
 
                 <p>
                     <label for="us-reg-email"><?php esc_html_e('Email', 'unsupervised-schedular'); ?></label>
-                    <?php if ($inviteValid && $invite !== null) : ?>
+                    <?php if ($inviteValid && $invite !== null && ! $invite->isGroup()) : ?>
                         <input type="email" id="us-reg-email" value="<?php echo esc_attr($invite->email); ?>" readonly>
                     <?php else : ?>
                         <input type="email" name="email" id="us-reg-email" autocomplete="email" required>