extend Post entity with feature slot

Adds nullable feature_slot column (unique) plus repo
findByFeatureSlot/findFeatured/update methods so admins can
pin a post into one of two slots.

backend/tests/Fakes/FakePostRepository.php

@@ -5,6 +5,7 @@ namespace Tests\Fakes;
 use App\Post\CreatePostDto;
 use App\Post\Post;
 use App\Post\PostRepository;
+use RuntimeException;
 
 class FakePostRepository implements PostRepository
 {
@@ -22,10 +23,11 @@ class FakePostRepository implements PostRepository
             title: $dto->title,
             body: $dto->body,
             createdAt: $dto->createdAt,
+            featureSlot: null,
         );
         $this->existingPosts[$id] = $post;
 
-        return $post;
+        return $this->copy($post);
     }
 
     public function find(int $id): ?Post
@@ -85,6 +87,54 @@ class FakePostRepository implements PostRepository
         unset($this->existingPosts[$id]);
     }
 
+    /**
+     * @throws RuntimeException
+     */
+    public function update(Post $post): Post
+    {
+        $id = $post->getId();
+        if (! isset($this->existingPosts[$id])) {
+            throw new RuntimeException(
+                "Post with id: $id does not exist"
+            );
+        }
+        $this->existingPosts[$id] = $post;
+
+        return $this->copy($post);
+    }
+
+    public function findByFeatureSlot(int $slot): ?Post
+    {
+        foreach ($this->existingPosts as $post) {
+            if ($post->getFeatureSlot() === $slot) {
+                return $this->copy($post);
+            }
+        }
+
+        return null;
+    }
+
+    /**
+     * @return Post[]
+     */
+    public function findFeatured(): array
+    {
+        $featured = [];
+        foreach ($this->existingPosts as $post) {
+            if ($post->isFeatured()) {
+                $featured[] = $this->copy($post);
+            }
+        }
+        usort(
+            $featured,
+            function (Post $left, Post $right) {
+                return $left->getFeatureSlot() <=> $right->getFeatureSlot();
+            },
+        );
+
+        return $featured;
+    }
+
     private function copy(Post $post): Post
     {
         return new Post(
@@ -93,6 +143,7 @@ class FakePostRepository implements PostRepository
             title: $post->getTitle(),
             body: $post->getBody(),
             createdAt: $post->getCreatedAt(),
+            featureSlot: $post->getFeatureSlot(),
         );
     }