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/TaxRegime.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Relations\BelongsToMany;
+use Illuminate\Database\Eloquent\Model;
+
+class TaxRegime extends Model
+{
+    use HasFactory;
+
+    public function taxPayers(): BelongsToMany
+    {
+        return $this->belongsToMany(TaxPayers::class);
+    }
+}