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/Plugin.php

@@ -51,10 +51,20 @@ class Plugin {
 			( new Installer() )->run();
 		}
 
-		$availability     = new AvailabilityRepository( $wpdb );
-		$bookings         = new BookingRepository( $wpdb );
-		$offerings        = new OfferingRepository( $wpdb );
-		$questions        = new QuestionRepository( $wpdb );
+		$availability = new AvailabilityRepository( $wpdb );
+		$bookings     = new BookingRepository( $wpdb );
+		$offerings    = new OfferingRepository( $wpdb );
+		$questions    = new QuestionRepository( $wpdb );
+
+		// One-time repair for sites where dbDelta left us_questions.offering_id
+		// NOT NULL (it does not reliably relax NULL-ability), which breaks
+		// account-scope registration questions. Guarded by its own flag rather
+		// than the version gate, since affected sites may already be on the
+		// current version. The flag is only set once the ALTER succeeds.
+		if ( '1' !== get_option( 'us_questions_offering_nullable', '' ) && $questions->ensureOfferingNullable() ) {
+			update_option( 'us_questions_offering_nullable', '1' );
+		}
+
 		$answers          = new AnswerRepository( $wpdb );
 		$policies         = new PolicyRepository( $wpdb );
 		$policyVersions   = new PolicyVersionRepository( $wpdb );