fix(api): hide sensitive fields by default

Add model-level hidden fields for secrets, tokens, keys, notification
credentials, deployment logs, and environment values. Allow explicit
read:sensitive API access to reveal gated private keys and deployment logs,
and cover the behavior with feature and unit tests.

app/Models/PrivateKey.php

@@ -41,6 +41,10 @@ class PrivateKey extends BaseModel
         'fingerprint',
     ];
 
+    protected $hidden = [
+        'private_key',
+    ];
+
     protected $casts = [
         'private_key' => 'encrypted',
     ];