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/RestRegistrar.php

@@ -15,6 +15,7 @@ use Unsupervised\Schedular\Policy\PolicyService;
 use Unsupervised\Schedular\Policy\PolicyVersionRepository;
 use Unsupervised\Schedular\Registration\QuestionEndpoint;
 use Unsupervised\Schedular\Registration\QuestionRepository;
+use Unsupervised\Schedular\Registration\RegistrationGate;
 
 class RestRegistrar {
 
@@ -26,9 +27,9 @@ class RestRegistrar {
 	private QuestionEndpoint $questionEndpoint;
 	private PolicyEndpoint $policyEndpoint;
 
-	public function __construct( AvailabilityRepository $availability, BookingRepository $bookings, OfferingRepository $offerings, QuestionRepository $questions, PolicyRepository $policies, PolicyVersionRepository $policyVersions, PolicyService $policyService ) {
+	public function __construct( AvailabilityRepository $availability, BookingRepository $bookings, OfferingRepository $offerings, QuestionRepository $questions, PolicyRepository $policies, PolicyVersionRepository $policyVersions, PolicyService $policyService, RegistrationGate $gate ) {
 		$this->availabilityEndpoint = new AvailabilityEndpoint( $availability );
-		$this->bookingEndpoint      = new BookingEndpoint( $availability, $bookings );
+		$this->bookingEndpoint      = new BookingEndpoint( $availability, $bookings, $offerings, $gate );
 		$this->offeringEndpoint     = new OfferingEndpoint( $offerings );
 		$this->questionEndpoint     = new QuestionEndpoint( $questions, $offerings );
 		$this->policyEndpoint       = new PolicyEndpoint( $policies, $policyVersions, $policyService );