Add Registration Questions domain (per-offering intake forms)

Implements #5: studio admin / instructors author intake questions scoped
per offering; answers are stored against a lesson or group enrolment via a
polymorphic registration reference.

- src/Registration/: Question + Answer value objects, QuestionRepository
  and AnswerRepository, QuestionEndpoint (REST), QuestionController +
  templates/admin/questions.php (Offerings -> Questions submenu)
- us_questions and us_question_answers tables in Schema.php
- REST: public GET /offerings/{id}/questions; POST/PATCH/DELETE /questions
  gated by manage_questions + offering ownership (owner or studio admin)
- Field types text/textarea/select/checkbox; select options stored as JSON
- Wiring in Plugin, RestRegistrar, AdminMenu

AnswerRepository is built now and consumed by the booking/enrolment flow
in #3/#4.

Tests: tests/Unit/Registration/ (19 tests). composer test (63 total), cs,
and PHPStan level 6 all pass.

Refs #5

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

src/Registration/AnswerRepository.php

@@ -0,0 +1,57 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Registration;
+
+class AnswerRepository {
+
+	private string $table;
+
+	public function __construct( private \wpdb $db ) {
+		$this->table = $db->prefix . 'us_question_answers';
+	}
+
+	public function insert( Answer $answer ): int {
+		$this->db->insert(
+			$this->table,
+			[
+				'question_id'       => $answer->questionId,
+				'registration_type' => $answer->registrationType,
+				'registration_id'   => $answer->registrationId,
+				'student_id'        => $answer->studentId,
+				'answer_value'      => $answer->answerValue,
+				'created_at'        => current_time( 'mysql' ),
+			],
+			[ '%d', '%s', '%d', '%d', '%s', '%s' ]
+		);
+
+		return $this->db->insert_id;
+	}
+
+	/**
+	 * Persist a batch of answers for a single registration.
+	 *
+	 * @param list<Answer> $answers
+	 * @return list<int> Inserted answer IDs.
+	 */
+	public function insertMany( array $answers ): array {
+		return array_map( fn( Answer $answer ): int => $this->insert( $answer ), $answers );
+	}
+
+	/**
+	 * Find all answers attached to a registration (lesson or enrolment).
+	 *
+	 * @return list<Answer>
+	 */
+	public function findByRegistration( string $registrationType, int $registrationId ): array {
+		$rows = $this->db->get_results(
+			$this->db->prepare(
+				"SELECT * FROM {$this->table} WHERE registration_type = %s AND registration_id = %d ORDER BY id ASC",
+				$registrationType,
+				$registrationId
+			)
+		);
+
+		return array_map( Answer::fromRow( ... ), $rows ?? [] );
+	}
+}