feat: enterprise MFA with TOTP, email codes, backup codes, and trusted devices

Three verification methods (TOTP authenticator, email code, backup codes),
trusted device management with 30-day expiry, role-based enforcement for
super_admin and org_admin, admin reset capability, and full test coverage
(46 tests). Modifies login flow to support MFA challenge/response with
temporary session tokens stored in cache.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/app/Models/MfaBackupCode.php

@@ -0,0 +1,37 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+final class MfaBackupCode extends Model
+{
+    protected $fillable = [
+        'user_id',
+        'code_hash',
+        'used',
+        'used_at',
+    ];
+
+    protected function casts(): array
+    {
+        return [
+            'used' => 'boolean',
+            'used_at' => 'datetime',
+        ];
+    }
+
+    public function user(): BelongsTo
+    {
+        return $this->belongsTo(User::class);
+    }
+
+    public function scopeUnused(Builder $query): Builder
+    {
+        return $query->where('used', false);
+    }
+}