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,
+    ) {}
+}