Add Pushover support and avatar uploads directory setup

Introduces Pushover as a notification channel with validation and form handling in the group edit view. Also updates the bootstrap script to create and set permissions for the avatar uploads directory.

app/Controllers/NotificationGroupController.php

@@ -468,6 +468,7 @@ class NotificationGroupController extends Controller
             'discord' => 'Discord',
             'slack' => 'Slack',
             'mattermost' => 'Mattermost',
+            'pushover' => 'Pushover',
             'webhook' => 'Webhook'
         ];