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>

tests/Unit/Registration/AnswerTest.php

@@ -0,0 +1,57 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Tests\Unit\Registration;
+
+use Unsupervised\Schedular\Registration\Answer;
+use Unsupervised\Schedular\Tests\Unit\TestCase;
+
+class AnswerTest extends TestCase
+{
+    public function testConstructorAndProperties(): void
+    {
+        $answer = new Answer(3, Answer::REG_LESSON, 12, 5, 'Beginner', 99);
+
+        self::assertSame(3, $answer->questionId);
+        self::assertSame(Answer::REG_LESSON, $answer->registrationType);
+        self::assertSame(12, $answer->registrationId);
+        self::assertSame(5, $answer->studentId);
+        self::assertSame('Beginner', $answer->answerValue);
+        self::assertSame(99, $answer->id);
+    }
+
+    public function testFromRowMapsCorrectly(): void
+    {
+        $row = (object) [
+            'id'                => '99',
+            'question_id'       => '3',
+            'registration_type' => Answer::REG_ENROLLMENT,
+            'registration_id'   => '12',
+            'student_id'        => '5',
+            'answer_value'      => '1',
+        ];
+
+        $answer = Answer::fromRow($row);
+
+        self::assertSame(99, $answer->id);
+        self::assertSame(3, $answer->questionId);
+        self::assertSame(Answer::REG_ENROLLMENT, $answer->registrationType);
+        self::assertSame(12, $answer->registrationId);
+    }
+
+    public function testToArrayContainsExpectedKeys(): void
+    {
+        $answer = new Answer(3, Answer::REG_LESSON, 12, 5);
+        $arr    = $answer->toArray();
+
+        foreach (['id', 'question_id', 'registration_type', 'registration_id', 'student_id', 'answer_value'] as $key) {
+            self::assertArrayHasKey($key, $arr);
+        }
+    }
+
+    public function testValidRegistrationTypeConstants(): void
+    {
+        self::assertContains(Answer::REG_LESSON, Answer::VALID_REGISTRATION_TYPES);
+        self::assertContains(Answer::REG_ENROLLMENT, Answer::VALID_REGISTRATION_TYPES);
+    }
+}