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/app/Post/PostRepository.php

@@ -2,6 +2,8 @@
 
 namespace App\Post;
 
+use RuntimeException;
+
 interface PostRepository
 {
     public function create(CreatePostDto $dto): Post;
@@ -19,4 +21,16 @@ interface PostRepository
     public function findRecent(int $limit): array;
 
     public function delete(int $id): void;
+
+    /**
+     * @throws RuntimeException
+     */
+    public function update(Post $post): Post;
+
+    public function findByFeatureSlot(int $slot): ?Post;
+
+    /**
+     * @return Post[]
+     */
+    public function findFeatured(): array;
 }