Restructure src/ and tests/ from package-by-type to package-by-domain

All classes are now organised by domain (Availability, Booking, Auth).
Each domain package contains its value object, repository, admin controller,
REST endpoint, and any shortcode pages under a matching sub-namespace.
Cross-cutting wiring (Plugin, AdminMenu, RestRegistrar, ShortcodeRegistrar,
Schema) lives at src/ root. Tests mirror the domain structure.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Schema.php

@@ -0,0 +1,42 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular;
+
+class Schema {
+
+	/**
+	 * Returns CREATE TABLE statements for dbDelta.
+	 *
+	 * @return list<string>
+	 */
+	public static function tables( string $prefix, string $charset ): array {
+		return [
+			"CREATE TABLE {$prefix}us_availability (
+                id           BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
+                instructor_id BIGINT UNSIGNED NOT NULL,
+                start_dt     DATETIME        NOT NULL,
+                end_dt       DATETIME        NOT NULL,
+                is_booked    TINYINT(1)      NOT NULL DEFAULT 0,
+                created_at   DATETIME        NOT NULL,
+                PRIMARY KEY (id),
+                KEY instructor_id (instructor_id),
+                KEY start_dt (start_dt)
+            ) {$charset};",
+
+			"CREATE TABLE {$prefix}us_lessons (
+                id            BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
+                slot_id       BIGINT UNSIGNED NOT NULL,
+                student_id    BIGINT UNSIGNED NOT NULL,
+                instructor_id BIGINT UNSIGNED NOT NULL,
+                status        VARCHAR(20)     NOT NULL DEFAULT 'pending',
+                notes         TEXT,
+                created_at    DATETIME        NOT NULL,
+                PRIMARY KEY (id),
+                KEY slot_id (slot_id),
+                KEY student_id (student_id),
+                KEY instructor_id (instructor_id)
+            ) {$charset};",
+		];
+	}
+}