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/database/migrations/2026_05_06_000005_add_feature_slot_to_posts.php

@@ -0,0 +1,24 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('posts', function (Blueprint $table) {
+            $table->unsignedTinyInteger('feature_slot')->nullable();
+            $table->unique('feature_slot');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('posts', function (Blueprint $table) {
+            $table->dropUnique(['feature_slot']);
+            $table->dropColumn('feature_slot');
+        });
+    }
+};