refactor(core): centralize reserved system user identity

Signed-off-by: Sebastian Krupinski <krupinski01@gmail.com>

tests/php/Unit/Resource/SystemIdentityTest.php

@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+namespace KTXT\Unit\Resource;
+
+use KTXF\Mail\Provider\ProviderBaseInterface;
+use KTXF\Resource\SystemIdentity;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+
+final class SystemIdentityTest extends TestCase
+{
+    #[Test]
+    public function exposesTheReservedSystemUser(): void
+    {
+        self::assertSame('system', SystemIdentity::USER);
+    }
+
+    #[Test]
+    public function retainsTheLegacyMailAlias(): void
+    {
+        self::assertSame(SystemIdentity::USER, ProviderBaseInterface::USER_SYSTEM);
+    }
+}