Add Registration Questions domain (per-offering intake forms)

Implements #5: studio admin / instructors author intake questions scoped
per offering; answers are stored against a lesson or group enrolment via a
polymorphic registration reference.

- src/Registration/: Question + Answer value objects, QuestionRepository
  and AnswerRepository, QuestionEndpoint (REST), QuestionController +
  templates/admin/questions.php (Offerings -> Questions submenu)
- us_questions and us_question_answers tables in Schema.php
- REST: public GET /offerings/{id}/questions; POST/PATCH/DELETE /questions
  gated by manage_questions + offering ownership (owner or studio admin)
- Field types text/textarea/select/checkbox; select options stored as JSON
- Wiring in Plugin, RestRegistrar, AdminMenu

AnswerRepository is built now and consumed by the booking/enrolment flow
in #3/#4.

Tests: tests/Unit/Registration/ (19 tests). composer test (63 total), cs,
and PHPStan level 6 all pass.

Refs #5

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Plugin.php

@@ -7,6 +7,7 @@ use Unsupervised\Schedular\Auth\RoleManager;
 use Unsupervised\Schedular\Availability\AvailabilityRepository;
 use Unsupervised\Schedular\Booking\BookingRepository;
 use Unsupervised\Schedular\Offering\OfferingRepository;
+use Unsupervised\Schedular\Registration\QuestionRepository;
 
 class Plugin {
 
@@ -17,10 +18,11 @@ class Plugin {
 		$availability = new AvailabilityRepository( $wpdb );
 		$bookings     = new BookingRepository( $wpdb );
 		$offerings    = new OfferingRepository( $wpdb );
+		$questions    = new QuestionRepository( $wpdb );
 
 		( new RoleManager() )->register();
-		( new AdminMenu( $availability, $bookings, $offerings ) )->register();
-		( new RestRegistrar( $availability, $bookings, $offerings ) )->register();
+		( new AdminMenu( $availability, $bookings, $offerings, $questions ) )->register();
+		( new RestRegistrar( $availability, $bookings, $offerings, $questions ) )->register();
 		( new ShortcodeRegistrar() )->register();
 	}
 }