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/Registration/RegistrationGate.php

@@ -0,0 +1,120 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Registration;
+
+use Unsupervised\Schedular\Policy\AcceptanceRepository;
+use Unsupervised\Schedular\Policy\Policy;
+use Unsupervised\Schedular\Policy\PolicyAcceptance;
+use Unsupervised\Schedular\Policy\PolicyRepository;
+use Unsupervised\Schedular\Policy\PolicyVersionRepository;
+
+/**
+ * Shared registration gate: validates and records the intake-question answers
+ * and booking-scoped policy acceptances common to lesson bookings and group
+ * enrolments.
+ */
+class RegistrationGate {
+
+	public function __construct(
+		private QuestionRepository $questions,
+		private AnswerRepository $answers,
+		private PolicyRepository $policies,
+		private PolicyVersionRepository $versions,
+		private AcceptanceRepository $acceptances,
+	) {}
+
+	/**
+	 * Validate that required questions are answered and all booking-scoped
+	 * published policies are accepted. Returns null when the gate passes.
+	 *
+	 * @param array<int, string> $answers            question_id => answer value
+	 * @param list<int>          $acceptedVersionIds Accepted policy version IDs
+	 */
+	public function validate( int $offeringId, array $answers, array $acceptedVersionIds ): ?\WP_Error {
+		foreach ( $this->questions->findByOffering( $offeringId, true ) as $question ) {
+			if ( $question->isRequired && '' === trim( (string) ( $answers[ (int) $question->id ] ?? '' ) ) ) {
+				return new \WP_Error(
+					'missing_answer',
+					__( 'Please answer all required questions.', 'unsupervised-schedular' ),
+					[ 'status' => 400 ]
+				);
+			}
+		}
+
+		foreach ( $this->requiredPolicyVersionIds() as $versionId ) {
+			if ( ! in_array( $versionId, $acceptedVersionIds, true ) ) {
+				return new \WP_Error(
+					'policy_required',
+					__( 'You must accept all required policies.', 'unsupervised-schedular' ),
+					[ 'status' => 400 ]
+				);
+			}
+		}
+
+		return null;
+	}
+
+	/**
+	 * Persist answers and policy acceptances for a created registration.
+	 *
+	 * @param array<int, string> $answers            question_id => answer value
+	 * @param list<int>          $acceptedVersionIds Accepted policy version IDs
+	 */
+	public function record( string $registrationType, int $registrationId, int $studentId, int $offeringId, array $answers, array $acceptedVersionIds, ?string $ipAddress = null ): void {
+		foreach ( $this->questions->findByOffering( $offeringId, true ) as $question ) {
+			$value = (string) ( $answers[ (int) $question->id ] ?? '' );
+			if ( '' === $value ) {
+				continue;
+			}
+
+			$this->answers->insert(
+				new Answer(
+					questionId:       (int) $question->id,
+					registrationType: $registrationType,
+					registrationId:   $registrationId,
+					studentId:        $studentId,
+					answerValue:      $value,
+				)
+			);
+		}
+
+		foreach ( $this->requiredPolicyVersionIds() as $versionId ) {
+			if ( ! in_array( $versionId, $acceptedVersionIds, true ) ) {
+				continue;
+			}
+
+			$this->acceptances->insert(
+				new PolicyAcceptance(
+					policyVersionId:  $versionId,
+					studentId:        $studentId,
+					registrationType: $registrationType,
+					registrationId:   $registrationId,
+					ipAddress:        $ipAddress,
+				)
+			);
+		}
+	}
+
+	/**
+	 * Current published version IDs of every booking-scoped policy.
+	 *
+	 * @return list<int>
+	 */
+	private function requiredPolicyVersionIds(): array {
+		$ids = [];
+
+		foreach ( $this->policies->findForScope( Policy::SCOPE_BOOKING ) as $policy ) {
+			if ( null === $policy->currentVersionId ) {
+				continue;
+			}
+
+			$version = $this->versions->findById( $policy->currentVersionId );
+			if ( null !== $version && $version->isPublished() ) {
+				$ids[] = (int) $version->id;
+			}
+		}
+
+		return $ids;
+	}
+}