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/Availability/AvailabilityRepository.php

@@ -0,0 +1,111 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Availability;
+
+class AvailabilityRepository {
+
+	private string $table;
+
+	public function __construct( private \wpdb $db ) {
+		$this->table = $db->prefix . 'us_availability';
+	}
+
+	public function insert( AvailabilitySlot $slot ): int {
+		$this->db->insert(
+			$this->table,
+			[
+				'instructor_id' => $slot->instructorId,
+				'start_dt'      => $slot->startDt,
+				'end_dt'        => $slot->endDt,
+				'is_booked'     => 0,
+				'created_at'    => current_time( 'mysql' ),
+			],
+			[ '%d', '%s', '%s', '%d', '%s' ]
+		);
+
+		return $this->db->insert_id;
+	}
+
+	/**
+	 * Find unbooked slots, optionally filtered by instructor and date range.
+	 *
+	 * @return list<AvailabilitySlot>
+	 */
+	public function findAvailable( int $instructorId = 0, string $from = '', string $to = '' ): array {
+		$where  = [ 'is_booked = 0' ];
+		$params = [];
+
+		if ( $instructorId > 0 ) {
+			$where[]  = 'instructor_id = %d';
+			$params[] = $instructorId;
+		}
+
+		if ( '' !== $from ) {
+			$where[]  = 'start_dt >= %s';
+			$params[] = $from;
+		}
+
+		if ( '' !== $to ) {
+			$where[]  = 'end_dt <= %s';
+			$params[] = $to;
+		}
+
+		$whereClause = implode( ' AND ', $where );
+		$sql         = "SELECT * FROM {$this->table} WHERE {$whereClause} ORDER BY start_dt ASC";
+
+		$rows = $params
+			? $this->db->get_results( $this->db->prepare( $sql, $params ) )
+			: $this->db->get_results( $sql );
+
+		return array_map( AvailabilitySlot::fromRow( ... ), $rows ?? [] );
+	}
+
+	/**
+	 * Find all slots for an instructor (booked and unbooked).
+	 *
+	 * @return list<AvailabilitySlot>
+	 */
+	public function findByInstructor( int $instructorId ): array {
+		$rows = $this->db->get_results(
+			$this->db->prepare(
+				"SELECT * FROM {$this->table} WHERE instructor_id = %d ORDER BY start_dt ASC",
+				$instructorId
+			)
+		);
+
+		return array_map( AvailabilitySlot::fromRow( ... ), $rows ?? [] );
+	}
+
+	public function findById( int $id ): ?AvailabilitySlot {
+		$row = $this->db->get_row(
+			$this->db->prepare( "SELECT * FROM {$this->table} WHERE id = %d", $id )
+		);
+
+		return $row ? AvailabilitySlot::fromRow( $row ) : null;
+	}
+
+	public function markBooked( int $id ): bool {
+		return (bool) $this->db->update(
+			$this->table,
+			[ 'is_booked' => 1 ],
+			[ 'id' => $id ],
+			[ '%d' ],
+			[ '%d' ]
+		);
+	}
+
+	/**
+	 * Delete an unbooked slot. Returns false if the slot is already booked.
+	 */
+	public function delete( int $id ): bool {
+		return (bool) $this->db->delete(
+			$this->table,
+			[
+				'id'        => $id,
+				'is_booked' => 0,
+			],
+			[ '%d', '%d' ]
+		);
+	}
+}