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]>

CHANGELOG.md

@@ -21,10 +21,15 @@ each change under the current top section as you work.
 - The **Add/Edit Offering** form now shows only the fields relevant to the selected kind: the group-class settings (capacity, dates, times, enrolment/withdrawal deadlines, sessions, schedule note, invite-only) appear only for a group class, and the weekly-reservation option only for a private lesson.
 - Instructors can add students to any group class by hand from its details page (**Add students directly**), which now appears for public classes too, not just invite-only ones. This bypasses the enrolment deadline and capacity, so a student can be enrolled as a **late enrolment** after the class has closed to self-enrolment.
 - Studio admins and instructors can open a **lesson detail view** from the Scheduler and My Lessons lists, showing the offering booked, the policy versions the student accepted (with acceptance time and IP), and their intake answers. On My Lessons an instructor may only open their own lessons; the studio Scheduler may open any.
+- The **Student Registration** block's "registration is by invitation only" message is now customisable, under a new **Invitation-only notice** panel (shortcode: `invite_only_message`). Leaving it blank keeps the default wording.
 
 ### Changed
 - The student **upcoming lessons** panel now shows each booked offering's name and length beside the time, and lists only the soonest five lessons with a "Show all" reveal. The Scheduler and My Lessons week/list views likewise show the booked offering.
 
+### Fixed
+- Accepting an invitation now keeps the student signed in. Previously the registration form processed the submission after the page had started rendering, so the sign-in cookie was never sent and the new student was bounced back to the (logged-out) registration page; it is now handled before any output, and the student lands logged in.
+- Account-registration questions now save. On sites first installed before account-scope questions existed, the `us_questions.offering_id` column was left `NOT NULL` (the schema migration relied on `dbDelta`, which does not reliably relax a column to allow `NULL`), so saving an account question failed with "Column 'offering_id' cannot be null". A one-time, self-healing migration relaxes the column on the next load.
+
 ## [1.1.1]
 
 ### Fixed