test ListRecentPosts use case

5 cases: zero/negative limit -> BadRequest; empty repo -> [];
returns posts ordered newest-first; respects limit truncation.
fails red - ListRecentPosts class missing.

backend/tests/Unit/Post/UseCases/ListRecentPostsTest.php

@@ -0,0 +1,94 @@
+<?php
+
+namespace Tests\Unit\Post\UseCases;
+
+use App\Exceptions\BadRequestException;
+use App\Post\CreatePostDto;
+use App\Post\UseCases\ListRecentPosts\ListRecentPosts;
+use App\Post\UseCases\ListRecentPosts\ListRecentPostsRequest;
+use DateTimeImmutable;
+use DateTimeZone;
+use Tests\Fakes\FakePostRepository;
+use Tests\TestCase;
+
+class ListRecentPostsTest extends TestCase
+{
+    private FakePostRepository $postRepo;
+
+    private DateTimeImmutable $now;
+
+    private ListRecentPosts $useCase;
+
+    protected function setUp(): void
+    {
+        $this->now = new DateTimeImmutable(
+            '2026-05-06T12:00:00',
+            new DateTimeZone('UTC')
+        );
+        $this->postRepo = new FakePostRepository;
+        $this->useCase = new ListRecentPosts($this->postRepo);
+    }
+
+    private function seedPost(int $userId, string $title, string $offset): void
+    {
+        $this->postRepo->create(new CreatePostDto(
+            userId: $userId,
+            title: $title,
+            body: 'body',
+            createdAt: $this->now->modify($offset),
+        ));
+    }
+
+    public function test_zero_limit_throws_bad_request(): void
+    {
+        $this->expectException(BadRequestException::class);
+        $this->useCase->execute(new ListRecentPostsRequest(limit: 0));
+    }
+
+    public function test_negative_limit_throws_bad_request(): void
+    {
+        $this->expectException(BadRequestException::class);
+        $this->useCase->execute(new ListRecentPostsRequest(limit: -1));
+    }
+
+    public function test_empty_repo_returns_empty_array(): void
+    {
+        $posts = $this->useCase->execute(
+            new ListRecentPostsRequest(limit: 10)
+        );
+
+        $this->assertSame([], $posts);
+    }
+
+    public function test_returns_posts_ordered_newest_first(): void
+    {
+        $this->seedPost(1, 'oldest', '-2 days');
+        $this->seedPost(2, 'middle', '-1 day');
+        $this->seedPost(3, 'newest', '-1 hour');
+
+        $posts = $this->useCase->execute(
+            new ListRecentPostsRequest(limit: 10)
+        );
+
+        $this->assertCount(3, $posts);
+        $this->assertSame('newest', $posts[0]->getTitle());
+        $this->assertSame('middle', $posts[1]->getTitle());
+        $this->assertSame('oldest', $posts[2]->getTitle());
+    }
+
+    public function test_respects_limit(): void
+    {
+        $this->seedPost(1, 'first', '-3 days');
+        $this->seedPost(2, 'second', '-2 days');
+        $this->seedPost(3, 'third', '-1 day');
+        $this->seedPost(4, 'fourth', '-1 hour');
+
+        $posts = $this->useCase->execute(
+            new ListRecentPostsRequest(limit: 2)
+        );
+
+        $this->assertCount(2, $posts);
+        $this->assertSame('fourth', $posts[0]->getTitle());
+        $this->assertSame('third', $posts[1]->getTitle());
+    }
+}