feat: add admin seeder and is_admin factory state

Runs is_admin migration, adds AdminSeeder for the admin user, registers
it in DatabaseSeeder, adds admin() factory state to UserFactory, and
adds AdminAccessTest covering both forbidden and ok cases.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

database/seeders/DatabaseSeeder.php

@@ -13,7 +13,7 @@ class DatabaseSeeder extends Seeder
      */
     public function run(): void
     {
-        // User::factory(10)->create();
+        $this->call(AdminSeeder::class);
 
         User::factory()->create([
             'name' => 'Test User',