step 1, 2 and 3 of the implementation plan

app/Models/QuestionGroup.php

@@ -0,0 +1,50 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\HasMany;
+
+final class QuestionGroup extends Model
+{
+    /**
+     * Fillable attributes for mass assignment.
+     */
+    protected $fillable = [
+        'category_id',
+        'name',
+        'sort_order',
+        'description',
+        'scoring_instructions',
+    ];
+
+    /**
+     * Cast attributes to specific types.
+     */
+    protected function casts(): array
+    {
+        return [
+            'category_id' => 'integer',
+            'sort_order' => 'integer',
+        ];
+    }
+
+    /**
+     * Get the category that owns this question group.
+     */
+    public function category(): BelongsTo
+    {
+        return $this->belongsTo(Category::class);
+    }
+
+    /**
+     * Get all questions for this question group.
+     */
+    public function questions(): HasMany
+    {
+        return $this->hasMany(Question::class);
+    }
+}