feat: add notifications module with bell UI, event listeners, and cleanup job

In-app notification system with topbar bell icon, dropdown panel,
mark-as-read/dismiss actions, and scheduled cleanup of old read
notifications. Includes event listeners for user.created and
user.deleted events, authorization policy, and full test coverage.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

config/modules.php

@@ -12,5 +12,5 @@
  * Each entry must match a directory name under modules/ containing a module.php manifest.
  */
 return [
-    'enabled_modules' => ['addressbook', 'bookmarks'],
+    'enabled_modules' => ['addressbook', 'bookmarks', 'notifications'],
 ];