refactor(core): centralize reserved system user identity

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

shared/lib/Mail/Provider/ProviderBaseInterface.php

@@ -11,6 +11,7 @@ namespace KTXF\Mail\Provider;
 
 use KTXF\Mail\Service\ServiceBaseInterface;
 use KTXF\Resource\Provider\ResourceProviderBaseInterface;
+use KTXF\Resource\SystemIdentity;
 
 /**
  * Provider Base Interface
@@ -24,15 +25,9 @@ interface ProviderBaseInterface extends ResourceProviderBaseInterface{
     public const JSON_TYPE = 'mail:provider';
 
     /**
-     * Reserved user identifier for system-owned mail services
-     *
-     * Services stored under this user id belong to the tenant itself rather
-     * than any real user (e.g. accounts used to deliver system-generated
-     * messages such as verification codes and password resets).
-     *
-     * @since 2026.07.01
+     * @deprecated Use SystemIdentity::USER.
      */
-    public const USER_SYSTEM = 'system';
+    public const USER_SYSTEM = SystemIdentity::USER;
     
     /**
      * Finds a service that handles a specific email address