add Post entity, dto, repository interface

Post: id, userId (fk -> User), title, body, createdAt as
DateTimeImmutable. CreatePostDto readonly with explicit
createdAt (use case supplies it via Clock; entity remains pure).
PostRepository: create, find, findByUserId, findRecent (limit),
delete.

backend/app/Post/PostRepository.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace App\Post;
+
+interface PostRepository
+{
+    public function create(CreatePostDto $dto): Post;
+
+    public function find(int $id): ?Post;
+
+    /**
+     * @return Post[]
+     */
+    public function findByUserId(int $userId): array;
+
+    /**
+     * @return Post[]
+     */
+    public function findRecent(int $limit): array;
+
+    public function delete(int $id): void;
+}