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/Policy/PolicyEndpoint.php

@@ -69,12 +69,19 @@ class PolicyEndpoint {
 	}
 
 	/**
-	 * Public: the current published version of every policy (the registration gate).
+	 * Public: the current published version of every policy (the registration
+	 * gate). Pass `?scope=signup|booking` to limit to that gate (includes
+	 * `both`-scoped policies).
 	 */
-	public function index(): \WP_REST_Response {
+	public function index( \WP_REST_Request $request ): \WP_REST_Response {
+		$scope    = (string) $request->get_param( 'scope' );
+		$policies = in_array( $scope, [ Policy::SCOPE_SIGNUP, Policy::SCOPE_BOOKING ], true )
+			? $this->policies->findForScope( $scope )
+			: $this->policies->findAll();
+
 		$out = [];
 
-		foreach ( $this->policies->findAll() as $policy ) {
+		foreach ( $policies as $policy ) {
 			if ( null === $policy->currentVersionId ) {
 				continue;
 			}