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>

modules/notifications/lib/Module/Notifications/Providers/NotificationsLayoutProvider.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace MintyPHP\Module\Notifications\Providers;
+
+use MintyPHP\App\AppContainer;
+use MintyPHP\App\Module\Contracts\LayoutContextProvider;
+
+final class NotificationsLayoutProvider implements LayoutContextProvider
+{
+    public function provide(array $session, AppContainer $container): array
+    {
+        return [
+            'notifications.nav' => [
+                'unread_count' => (int) ($session['module.notifications.unread_count'] ?? 0),
+            ],
+        ];
+    }
+}