Make the SECRETS_KEY test able to fail

The Task 2 reviewer rewired the model to Laravel's APP_KEY-based Crypt
facade and every test stayed green. Laravel's encrypter is a container
singleton resolved once from app.key, so config()->set('app.key') never
rebuilds it and the rotation the test performed was invisible.

Replaced with a positive proof — rotating SECRETS_KEY must break
decryption — plus the APP_KEY case with forgetInstance(), which is what
makes that direction mean anything.

docs/superpowers/plans/2026-07-27-mailboxes.md

@@ -256,11 +256,30 @@ it('stores the password encrypted and never in the clear', function () {
         ->and($box->fresh()->password)->toBe('sehr-geheim');
 });
 
-it('uses SECRETS_KEY, not APP_KEY — rotating APP_KEY leaves mail working', function () {
+it('is keyed to SECRETS_KEY — rotating it makes the password unreadable', function () {
+    // The POSITIVE proof, and the one that discriminates. Without it the pair
+    // below passes even against an APP_KEY implementation.
+    $box = Mailbox::factory()->create(['password' => 'geheim']);
+
+    config()->set('admin_access.secrets_key', 'base64:'.base64_encode(random_bytes(32)));
+
+    expect(fn () => $box->fresh()->password)
+        ->toThrow(Illuminate\Contracts\Encryption\DecryptException::class);
+});
+
+it('is NOT keyed to APP_KEY — rotating that leaves mail working', function () {
     $box = Mailbox::factory()->create(['password' => 'bleibt-lesbar']);
 
     config()->set('app.key', 'base64:'.base64_encode(random_bytes(32)));
 
+    // forgetInstance is what makes this test mean anything. Laravel's encrypter
+    // is a container singleton resolved once from whatever app.key held at the
+    // time; config()->set() afterwards mutates the array and never rebuilds it.
+    // Without this line the test passes even when the model is rewired to
+    // Laravel's own APP_KEY-based Crypt facade — proven by mutation during the
+    // Task 2 review, where every test stayed green through exactly that change.
+    app()->forgetInstance('encrypter');
+
     expect($box->fresh()->password)->toBe('bleibt-lesbar');
 });