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/Registration/QuestionRepository.php

@@ -106,4 +106,26 @@ class QuestionRepository {
 			[ '%d' ]
 		);
 	}
+
+	/**
+	 * Relax `offering_id` to allow NULL for account-scope questions (which are
+	 * not tied to an offering).
+	 *
+	 * The account-questions feature (v1.1.0) made the column nullable in the
+	 * schema, but dbDelta does not reliably change a column from NOT NULL to
+	 * NULL, so sites created before then keep the old NOT NULL column and reject
+	 * account-scope inserts with "Column 'offering_id' cannot be null". This
+	 * MODIFY is idempotent — re-applying the nullable definition is a no-op.
+	 *
+	 * @return bool True when the statement ran (or was already applied), false
+	 *              if it could not be prepared or the query failed.
+	 */
+	public function ensureOfferingNullable(): bool {
+		$sql = $this->db->prepare(
+			'ALTER TABLE %i MODIFY offering_id BIGINT UNSIGNED NULL DEFAULT NULL',
+			$this->table
+		);
+
+		return null !== $sql && false !== $this->db->query( $sql );
+	}
 }