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]>

assets/js/blocks.js

@@ -5,7 +5,7 @@
     const { registerBlockType } = wp.blocks;
     const { createElement: el, useState, useEffect } = wp.element;
     const { useBlockProps, InspectorControls } = wp.blockEditor;
-    const { PanelBody, SelectControl, ToggleControl } = wp.components;
+    const { PanelBody, SelectControl, ToggleControl, TextareaControl } = wp.components;
     const { useSelect } = wp.data;
     const apiFetch = wp.apiFetch;
     const ServerSideRender = wp.serverSideRender;
@@ -151,18 +151,31 @@
             shortcode: 'us_student_register',
             attributes: {
                 loginPageId: { type: 'number', default: 0 },
+                inviteOnlyMessage: { type: 'string', default: '' },
             },
-            inspector: (attributes, setAttributes) => el(
-                PanelBody,
-                { title: __('After email confirmation', 'unsupervised-schedular') },
-                el(PageSelect, {
-                    label: __('Sign-in page', 'unsupervised-schedular'),
-                    help: __('Where the sign-in link shown after a student confirms their email address sends them.', 'unsupervised-schedular'),
-                    defaultLabel: __('WordPress login screen', 'unsupervised-schedular'),
-                    value: attributes.loginPageId,
-                    onChange: (loginPageId) => setAttributes({ loginPageId }),
-                })
-            ),
+            inspector: (attributes, setAttributes) => [
+                el(
+                    PanelBody,
+                    { title: __('After email confirmation', 'unsupervised-schedular'), key: 'confirmation' },
+                    el(PageSelect, {
+                        label: __('Sign-in page', 'unsupervised-schedular'),
+                        help: __('Where the sign-in link shown after a student confirms their email address sends them.', 'unsupervised-schedular'),
+                        defaultLabel: __('WordPress login screen', 'unsupervised-schedular'),
+                        value: attributes.loginPageId,
+                        onChange: (loginPageId) => setAttributes({ loginPageId }),
+                    })
+                ),
+                el(
+                    PanelBody,
+                    { title: __('Invitation-only notice', 'unsupervised-schedular'), key: 'invite-only' },
+                    el(TextareaControl, {
+                        label: __('Message', 'unsupervised-schedular'),
+                        help: __('Shown when registration is invite-only and the visitor has no valid invite link. Leave blank to use the default wording.', 'unsupervised-schedular'),
+                        value: attributes.inviteOnlyMessage,
+                        onChange: (inviteOnlyMessage) => setAttributes({ inviteOnlyMessage }),
+                    })
+                ),
+            ],
         },
         {
             name: 'us-scheduler/group-classes',