fix: add missing sanctum auth guard config and register driver

The main branch was using auth:sanctum middleware on routes but
config/auth.php had no sanctum guard defined. This caused:

  Auth guard [sanctum] is not defined.

Fix:
1. Add the sanctum guard entry to config/auth.php with
   driver=sanctum and provider=users
2. Register the sanctum guard driver in AppServiceProvider::boot()
   as a safety net via Auth::resolved()

config/auth.php

@@ -44,6 +44,10 @@ return [
             'driver' => 'jwt',
             'provider' => 'users',
         ],
+        'sanctum' => [
+            'driver' => 'sanctum',
+            'provider' => 'users',
+        ],
     ],
 
     /*