add Comment persistence: model, migration, eloquent + fake repo

backend/database/migrations/2026_05_06_000004_create_comments_table.php

@@ -0,0 +1,28 @@
+<?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::create('comments', function (Blueprint $table) {
+            $table->id();
+            $table->foreignId('post_id')
+                ->constrained('posts')
+                ->cascadeOnDelete();
+            $table->foreignId('user_id')
+                ->constrained('users')
+                ->cascadeOnDelete();
+            $table->text('body');
+            $table->dateTime('created_at')->index();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('comments');
+    }
+};