Initial commit: Horux Backend API

- API REST para gestion de facturas electronicas mexicanas (CFDI)
- Laravel 9 con autenticacion OAuth 2.0 (Passport)
- Integracion con Syntage, Clerk y Facturama
- 30 modelos Eloquent, 39 controladores
- Documentacion completa en /docs

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/Models/TaxRate.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+
+class TaxRate extends Model
+{
+    use HasFactory;
+
+    /**
+     * The attributes that are mass assignable.
+     *
+     * @var array<int, string>
+     */
+    protected $fillable = [
+        'id',
+        'tax_id',
+        'description'
+    ];
+
+}