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/Booking/BookingEndpoint.php

@@ -5,12 +5,17 @@ namespace Unsupervised\Schedular\Booking;
 
 use Unsupervised\Schedular\Availability\AvailabilityRepository;
 use Unsupervised\Schedular\Auth\RoleManager;
+use Unsupervised\Schedular\Offering\OfferingRepository;
+use Unsupervised\Schedular\Policy\PolicyAcceptance;
+use Unsupervised\Schedular\Registration\RegistrationGate;
 
 class BookingEndpoint {
 
 	public function __construct(
 		private AvailabilityRepository $availability,
 		private BookingRepository $bookings,
+		private OfferingRepository $offerings,
+		private RegistrationGate $gate,
 	) {}
 
 	public function registerRoutes( string $route_namespace ): void {
@@ -28,12 +33,28 @@ class BookingEndpoint {
 					'callback'            => [ $this, 'book' ],
 					'permission_callback' => [ $this, 'canBook' ],
 					'args'                => [
-						'slot_id' => [
+						'slot_id'                     => [
 							'type'              => 'integer',
 							'required'          => true,
 							'sanitize_callback' => 'absint',
 						],
-						'notes'   => [
+						'offering_id'                 => [
+							'type'    => 'integer',
+							'default' => 0,
+						],
+						'recurrence'                  => [
+							'type'    => 'string',
+							'default' => 'single',
+						],
+						'answers'                     => [
+							'type'    => 'object',
+							'default' => [],
+						],
+						'accepted_policy_version_ids' => [
+							'type'    => 'array',
+							'default' => [],
+						],
+						'notes'                       => [
 							'type'              => 'string',
 							'default'           => '',
 							'sanitize_callback' => 'sanitize_textarea_field',
@@ -84,26 +105,83 @@ class BookingEndpoint {
 			return new \WP_Error( 'slot_taken', __( 'This slot is already booked.', 'unsupervised-schedular' ), [ 'status' => 409 ] );
 		}
 
-		$notes  = (string) $request->get_param( 'notes' );
-		$lesson = new Lesson(
+		$offeringId = absint( $request->get_param( 'offering_id' ) );
+		if ( 0 === $offeringId ) {
+			$offeringId = (int) ( $slot->offeringId ?? 0 );
+		}
+		if ( $offeringId > 0 && null === $this->offerings->findById( $offeringId ) ) {
+			return new \WP_Error( 'invalid_offering', __( 'Offering not found.', 'unsupervised-schedular' ), [ 'status' => 400 ] );
+		}
+
+		$answers            = $this->answers( $request );
+		$acceptedVersionIds = array_map( 'absint', (array) $request->get_param( 'accepted_policy_version_ids' ) );
+
+		$gateError = $this->gate->validate( $offeringId, $answers, $acceptedVersionIds );
+		if ( $gateError instanceof \WP_Error ) {
+			return $gateError;
+		}
+
+		$studentId  = get_current_user_id();
+		$notes      = (string) $request->get_param( 'notes' );
+		$recurrence = Lesson::RECURRENCE_WEEKLY === $request->get_param( 'recurrence' )
+			? Lesson::RECURRENCE_WEEKLY
+			: Lesson::RECURRENCE_SINGLE;
+
+		$template = new Lesson(
 			slotId:       $slotId,
-			studentId:    get_current_user_id(),
+			studentId:    $studentId,
 			instructorId: $slot->instructorId,
+			offeringId:   $offeringId > 0 ? $offeringId : null,
+			recurrence:   $recurrence,
 			notes:        '' !== $notes ? $notes : null,
 		);
 
-		$id = $this->bookings->insert( $lesson );
-		$this->availability->markBooked( $slotId );
+		// Weekly reservation across the slot's recurring group; otherwise a single lesson.
+		if ( Lesson::RECURRENCE_WEEKLY === $recurrence && null !== $slot->recurrenceGroup ) {
+			$slotIds = array_map( static fn( $s ): int => (int) $s->id, $this->availability->findUnbookedInGroup( $slot->recurrenceGroup ) );
+			$ids     = $this->bookings->insertSeries( $template, $slotIds );
+			foreach ( $slotIds as $reservedSlotId ) {
+				$this->availability->markBooked( $reservedSlotId );
+			}
+			$anchorId = $ids[0] ?? 0;
+		} else {
+			$anchorId = $this->bookings->insert( $template );
+			$this->availability->markBooked( $slotId );
+			$ids = [ $anchorId ];
+		}
+
+		$this->gate->record( PolicyAcceptance::REG_LESSON, $anchorId, $studentId, $offeringId, $answers, $acceptedVersionIds, $this->clientIp() );
 
 		return new \WP_REST_Response(
 			[
-				'id'     => $id,
+				'ids'    => $ids,
 				'status' => Lesson::STATUS_PENDING,
 			],
 			201
 		);
 	}
 
+	/**
+	 * Extract a question_id => value map from the request.
+	 *
+	 * @return array<int, string>
+	 */
+	private function answers( \WP_REST_Request $request ): array {
+		$out = [];
+		foreach ( (array) $request->get_param( 'answers' ) as $questionId => $value ) {
+			$out[ (int) $questionId ] = sanitize_text_field( (string) $value );
+		}
+
+		return $out;
+	}
+
+	private function clientIp(): ?string {
+		// phpcs:ignore WordPress.Security.ValidatedSanitizedInput.InputNotSanitized, WordPress.Security.ValidatedSanitizedInput.MissingUnslash -- IP stored verbatim for audit.
+		$ip = sanitize_text_field( wp_unslash( $_SERVER['REMOTE_ADDR'] ?? '' ) );
+
+		return '' !== $ip ? $ip : null;
+	}
+
 	public function updateStatus( \WP_REST_Request $request ): \WP_REST_Response|\WP_Error {
 		$id     = absint( $request->get_param( 'id' ) );
 		$lesson = $this->bookings->findById( $id );