Fix invite sign-in persistence, add invite-only text option, repair account questions

Three registration fixes reported from live use:

- Accepting an invite now keeps the student signed in. The form was
  processed inside render() during the_content, so wp_set_auth_cookie()
  ran after headers were sent and the cookie never persisted — the new
  student was bounced back to the logged-out registration page. The
  submission is now handled on template_redirect (before output) with a
  post/redirect/get, so the cookie sticks and the student lands logged in.

- The "registration is by invitation only" message is now customisable via
  a new block attribute (inviteOnlyMessage / shortcode invite_only_message),
  falling back to the default wording when blank.

- Account-registration questions save again. dbDelta does not reliably
  relax a column from NOT NULL to NULL, so sites created before account-
  scope questions kept us_questions.offering_id NOT NULL and rejected
  account inserts ("Column 'offering_id' cannot be null"). A one-time,
  self-healing migration (guarded by its own option, not the version gate)
  re-applies the nullable definition on next load.

composer test, composer lint, composer cs all pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

templates/frontend/register-page.php

@@ -12,6 +12,7 @@ if (! defined('ABSPATH')) {
  * @var bool   $inviteValid Whether $invite can still be redeemed — only then is the email fixed.
  * @var string $token Raw invite token from the request (only its hash is stored).
  * @var bool   $canRegister
+ * @var string $inviteOnlyMessage Text shown when registration is closed and no valid invite is present.
  * @var bool   $open   Whether open (self-approval) registration is enabled.
  * @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'.
@@ -74,7 +75,7 @@ $renderQuestionField = static function (Question $question): void {
         <?php endif; ?>
 
         <?php if (! $canRegister) : ?>
-            <p><?php esc_html_e('Registration is by invitation only. Please use the link from your invitation email, or contact the studio.', 'unsupervised-schedular'); ?></p>
+            <p><?php echo esc_html($inviteOnlyMessage); ?></p>
         <?php else : ?>
             <?php if ($error !== '') : ?>
                 <p class="us-error" role="alert"><?php echo esc_html($error); ?></p>