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]>

tests/Unit/Registration/QuestionRepositoryTest.php

@@ -212,4 +212,28 @@ class QuestionRepositoryTest extends TestCase
 
         self::assertTrue($this->repo->delete(4));
     }
+
+    public function testEnsureOfferingNullableRunsAlterAndReportsSuccess(): void
+    {
+        $this->db->shouldReceive('prepare')
+            ->once()
+            ->with(Mockery::pattern('/ALTER TABLE %i MODIFY offering_id .*NULL/'), 'wp_us_questions')
+            ->andReturn('ALTER TABLE `wp_us_questions` MODIFY offering_id BIGINT UNSIGNED NULL DEFAULT NULL');
+
+        $this->db->shouldReceive('query')
+            ->once()
+            ->with('ALTER TABLE `wp_us_questions` MODIFY offering_id BIGINT UNSIGNED NULL DEFAULT NULL')
+            ->andReturn(0);
+
+        // A successful DDL query returns 0 rows affected (not false).
+        self::assertTrue($this->repo->ensureOfferingNullable());
+    }
+
+    public function testEnsureOfferingNullableReportsFailureWhenQueryFails(): void
+    {
+        $this->db->shouldReceive('prepare')->once()->andReturn('ALTER ...');
+        $this->db->shouldReceive('query')->once()->andReturn(false);
+
+        self::assertFalse($this->repo->ensureOfferingNullable());
+    }
 }