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]>

src/ShortcodeRegistrar.php

@@ -23,6 +23,9 @@ class ShortcodeRegistrar {
 		add_shortcode( 'us_student_login', self::shortcode( [ $this->loginPage, 'render' ] ) );
 		add_shortcode( 'us_student_register', self::shortcode( [ $this->registrationPage, 'render' ] ) );
 		add_shortcode( 'us_group_classes', self::shortcode( [ $this->groupClassPage, 'render' ] ) );
+		// Process registration submissions before output so the invite branch's
+		// auth cookie is actually sent (render() runs too late, during the_content).
+		add_action( 'template_redirect', [ $this->registrationPage, 'maybeHandleSubmit' ] );
 		add_action( 'template_redirect', [ $this->registrationPage, 'maybeRedirectToRegistrationPage' ] );
 		add_action( 'wp_enqueue_scripts', [ $this, 'enqueueAssets' ] );
 	}