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/Post.php

@@ -12,6 +12,7 @@ class Post
         private string $title,
         private string $body,
         private DateTimeImmutable $createdAt,
+        private ?int $featureSlot,
     ) {}
 
     public function getId(): int
@@ -38,4 +39,14 @@ class Post
     {
         return $this->createdAt;
     }
+
+    public function getFeatureSlot(): ?int
+    {
+        return $this->featureSlot;
+    }
+
+    public function isFeatured(): bool
+    {
+        return $this->featureSlot !== null;
+    }
 }