feat(sync) : add DTOs and SyncStrategyInterface

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

src/Service/Sync/SyncStrategyInterface.php

@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Service\Sync;
+
+use App\DTO\SyncConfirmation;
+use App\DTO\SyncExecutionResult;
+use App\DTO\SyncPreviewResult;
+use App\Entity\ModelType;
+
+interface SyncStrategyInterface
+{
+    public function supports(ModelType $modelType): bool;
+
+    /**
+     * Compute diff between proposed structure and current items' slots.
+     * Does NOT persist anything.
+     */
+    public function preview(ModelType $modelType, array $newStructure): SyncPreviewResult;
+
+    /**
+     * Apply sync: compare current skeleton requirements (already persisted)
+     * with items' slots and add/remove as needed.
+     */
+    public function execute(ModelType $modelType, SyncConfirmation $confirmation): SyncExecutionResult;
+}