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/CreatePostDto.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace App\Post;
+
+use DateTimeImmutable;
+
+readonly class CreatePostDto
+{
+    public function __construct(
+        public int $userId,
+        public string $title,
+        public string $body,
+        public DateTimeImmutable $createdAt,
+    ) {}
+}

backend/app/Post/Post.php

@@ -0,0 +1,41 @@
+<?php
+
+namespace App\Post;
+
+use DateTimeImmutable;
+
+class Post
+{
+    public function __construct(
+        private int $id,
+        private int $userId,
+        private string $title,
+        private string $body,
+        private DateTimeImmutable $createdAt,
+    ) {}
+
+    public function getId(): int
+    {
+        return $this->id;
+    }
+
+    public function getUserId(): int
+    {
+        return $this->userId;
+    }
+
+    public function getTitle(): string
+    {
+        return $this->title;
+    }
+
+    public function getBody(): string
+    {
+        return $this->body;
+    }
+
+    public function getCreatedAt(): DateTimeImmutable
+    {
+        return $this->createdAt;
+    }
+}

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;
+}