Add Mattermost notification channel support

Introduces Mattermost as a new notification channel, including backend service integration, controller validation, UI form fields, and updates to channel type enums in the database schema and migrations. This enables users to configure and send notifications via Mattermost webhooks.

app/Services/NotificationService.php

@@ -6,6 +6,7 @@ use App\Services\Channels\EmailChannel;
 use App\Services\Channels\TelegramChannel;
 use App\Services\Channels\DiscordChannel;
 use App\Services\Channels\SlackChannel;
+use App\Services\Channels\MattermostChannel;
 use App\Services\Channels\WebhookChannel;
 
 class NotificationService
@@ -19,6 +20,7 @@ class NotificationService
             'telegram' => new TelegramChannel(),
             'discord' => new DiscordChannel(),
             'slack' => new SlackChannel(),
+            'mattermost' => new MattermostChannel(),
             'webhook' => new WebhookChannel(),
         ];
     }