Add invite-only registration mode

Support configurable on, invite-only, and off registration modes, including case-insensitive invitation checks and test coverage.

app/Enums/RegistrationMode.php

@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Enums;
+
+enum RegistrationMode: string
+{
+    case On = 'on';
+    case InviteOnly = 'invite-only';
+    case Off = 'off';
+
+    public static function fromConfig(mixed $value): self
+    {
+        if ($value === true) {
+            return self::On;
+        }
+
+        if ($value === false || $value === null) {
+            return self::Off;
+        }
+
+        return match (strtolower(trim((string) $value))) {
+            '1', 'on', 'true' => self::On,
+            'invite', 'invite-only' => self::InviteOnly,
+            default => self::Off,
+        };
+    }
+}