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/Schema.php

@@ -60,6 +60,35 @@ class Schema {
                 KEY kind (kind),
                 KEY is_active (is_active)
             ) {$charset};",
+
+			"CREATE TABLE {$prefix}us_questions (
+                id          BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
+                offering_id BIGINT UNSIGNED NOT NULL,
+                label       VARCHAR(255)    NOT NULL,
+                field_type  VARCHAR(20)     NOT NULL DEFAULT 'text',
+                options     TEXT,
+                is_required TINYINT(1)      NOT NULL DEFAULT 0,
+                sort_order  INT             NOT NULL DEFAULT 0,
+                is_active   TINYINT(1)      NOT NULL DEFAULT 1,
+                created_at  DATETIME        NOT NULL,
+                PRIMARY KEY (id),
+                KEY offering_id (offering_id),
+                KEY is_active (is_active)
+            ) {$charset};",
+
+			"CREATE TABLE {$prefix}us_question_answers (
+                id                BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
+                question_id       BIGINT UNSIGNED NOT NULL,
+                registration_type VARCHAR(20)     NOT NULL,
+                registration_id   BIGINT UNSIGNED NOT NULL,
+                student_id        BIGINT UNSIGNED NOT NULL,
+                answer_value      TEXT,
+                created_at        DATETIME        NOT NULL,
+                PRIMARY KEY (id),
+                KEY question_id (question_id),
+                KEY registration (registration_type, registration_id),
+                KEY student_id (student_id)
+            ) {$charset};",
 		];
 	}
 }