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/factories/UserFactory.php

@@ -57,4 +57,12 @@ class UserFactory extends Factory
             'two_factor_confirmed_at' => now(),
         ]);
     }
+
+    /**
+     * Indicate that the user is an admin.
+     */
+    public function admin(): static
+    {
+        return $this->state(['is_admin' => true]);
+    }
 }