Add lesson booking registration flow (offering, questions, policies)

Implements #3: students register for a private lesson by picking a slot,
answering the offering's intake questions, and accepting booking-scoped
policies. Payment is a clean seam for #7 (lessons land pending; payment_id
null; instructor confirms via PATCH /bookings/{id}/status).

- Schema: us_lessons += offering_id, recurrence, series_id, payment_id.
- Lesson: new fields + recurrence constants.
- BookingRepository::insertSeries() builds a weekly series sharing a
  series_id; AvailabilityRepository::findUnbookedInGroup() reserves a group.
- RegistrationGate (src/Registration/): validate + record intake answers and
  booking-scoped policy acceptances. Reused by group enrolment (#4).
- BookingEndpoint::book(): offering_id, recurrence, answers,
  accepted_policy_version_ids; single or weekly; records answers/acceptances
  (type lesson).
- GET /policies?scope=booking filter.
- Front-end booking.js: slot -> questions + policies -> submit.
- Wiring: RegistrationGate built in Plugin, passed via RestRegistrar.
- Test-only WP_Error stub in tests/bootstrap.php for gate testing.

Refs #3

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

src/Plugin.php

@@ -12,7 +12,9 @@ use Unsupervised\Schedular\Policy\AcceptanceRepository;
 use Unsupervised\Schedular\Policy\PolicyRepository;
 use Unsupervised\Schedular\Policy\PolicyService;
 use Unsupervised\Schedular\Policy\PolicyVersionRepository;
+use Unsupervised\Schedular\Registration\AnswerRepository;
 use Unsupervised\Schedular\Registration\QuestionRepository;
+use Unsupervised\Schedular\Registration\RegistrationGate;
 
 class Plugin {
 
@@ -20,19 +22,21 @@ class Plugin {
 		load_plugin_textdomain( 'unsupervised-schedular', false, dirname( plugin_basename( USC_PLUGIN_FILE ) ) . '/languages' );
 
 		global $wpdb;
-		$availability   = new AvailabilityRepository( $wpdb );
-		$bookings       = new BookingRepository( $wpdb );
-		$offerings      = new OfferingRepository( $wpdb );
-		$questions      = new QuestionRepository( $wpdb );
-		$policies       = new PolicyRepository( $wpdb );
-		$policyVersions = new PolicyVersionRepository( $wpdb );
-		$policyService  = new PolicyService( $policies, $policyVersions );
-		$acceptances    = new AcceptanceRepository( $wpdb );
-		$invites        = new InviteRepository( $wpdb );
+		$availability     = new AvailabilityRepository( $wpdb );
+		$bookings         = new BookingRepository( $wpdb );
+		$offerings        = new OfferingRepository( $wpdb );
+		$questions        = new QuestionRepository( $wpdb );
+		$answers          = new AnswerRepository( $wpdb );
+		$policies         = new PolicyRepository( $wpdb );
+		$policyVersions   = new PolicyVersionRepository( $wpdb );
+		$policyService    = new PolicyService( $policies, $policyVersions );
+		$acceptances      = new AcceptanceRepository( $wpdb );
+		$invites          = new InviteRepository( $wpdb );
+		$registrationGate = new RegistrationGate( $questions, $answers, $policies, $policyVersions, $acceptances );
 
 		( new RoleManager() )->register();
 		( new AdminMenu( $availability, $bookings, $offerings, $questions, $policies, $policyVersions, $policyService, $invites ) )->register();
-		( new RestRegistrar( $availability, $bookings, $offerings, $questions, $policies, $policyVersions, $policyService ) )->register();
+		( new RestRegistrar( $availability, $bookings, $offerings, $questions, $policies, $policyVersions, $policyService, $registrationGate ) )->register();
 		( new ShortcodeRegistrar( $invites, $policies, $policyVersions, $acceptances ) )->register();
 	}
 }