Add all files

app/Models/Category.php

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+<?php
+
+namespace App\Models;
+
+use App\Observers\CategoryObserver;
+use Illuminate\Database\Eloquent\Attributes\ObservedBy;
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\HasMany;
+
+#[ObservedBy(CategoryObserver::class)]
+class Category extends Model
+{
+    use HasFactory;
+
+    protected $guarded = [];
+
+    /**
+     * Get the products of the category.
+     */
+    public function products(): HasMany
+    {
+        return $this->hasMany(Product::class);
+    }
+
+    /**
+     * Get the parent category of the category.
+     */
+    public function parent(): BelongsTo
+    {
+        return $this->belongsTo(Category::class, 'parent_id');
+    }
+
+    /**
+     * Get the children categories of the category.
+     */
+    public function children(): HasMany
+    {
+        return $this->hasMany(Category::class, 'parent_id');
+    }
+}