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 <[email protected]>

src/Registration/Answer.php

@@ -0,0 +1,53 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Registration;
+
+class Answer {
+
+	public const REG_LESSON     = 'lesson';
+	public const REG_ENROLLMENT = 'enrollment';
+
+	/**
+	 * Polymorphic registration targets an answer can attach to.
+	 *
+	 * @var list<string>
+	 */
+	public const VALID_REGISTRATION_TYPES = [ self::REG_LESSON, self::REG_ENROLLMENT ];
+
+	public function __construct(
+		public readonly int $questionId,
+		public readonly string $registrationType,
+		public readonly int $registrationId,
+		public readonly int $studentId,
+		public readonly ?string $answerValue = null,
+		public readonly ?int $id = null,
+	) {}
+
+	public static function fromRow( object $row ): self {
+		return new self(
+			questionId:       (int) $row->question_id,
+			registrationType: $row->registration_type,
+			registrationId:   (int) $row->registration_id,
+			studentId:        (int) $row->student_id,
+			answerValue:      $row->answer_value,
+			id:               (int) $row->id,
+		);
+	}
+
+	/**
+	 * Returns a plain array representation of the answer.
+	 *
+	 * @return array<string, mixed>
+	 */
+	public function toArray(): array {
+		return [
+			'id'                => $this->id,
+			'question_id'       => $this->questionId,
+			'registration_type' => $this->registrationType,
+			'registration_id'   => $this->registrationId,
+			'student_id'        => $this->studentId,
+			'answer_value'      => $this->answerValue,
+		];
+	}
+}