feat(sync) : add DTOs and SyncStrategyInterface

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

src/DTO/SyncExecutionResult.php

@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\DTO;
+
+use JsonSerializable;
+
+class SyncExecutionResult implements JsonSerializable
+{
+    public function __construct(
+        public readonly int $itemsUpdated,
+        public readonly array $additions = [],
+        public readonly array $deletions = [],
+        public readonly array $modifications = [],
+    ) {}
+
+    public function jsonSerialize(): array
+    {
+        return [
+            'itemsUpdated'  => $this->itemsUpdated,
+            'additions'     => $this->additions,
+            'deletions'     => $this->deletions,
+            'modifications' => $this->modifications,
+        ];
+    }
+}