fix: change Watchtower notification type from mattermost to shoutrrr

Watchtower was failing with "Unknown notification type 'mattermost'" because
'mattermost' is not a valid WATCHTOWER_NOTIFICATIONS value. When using
Shoutrrr URLs (WATCHTOWER_NOTIFICATION_URL), the notification type should
be set to 'shoutrrr' or left empty, as the service type is inferred from
the URL format (mattermost://...).

Changed WATCHTOWER_NOTIFICATIONS from 'mattermost' to 'shoutrrr' to fix
the fatal error that was occurring every minute.

The mattermost:// Shoutrrr URL in WATCHTOWER_NOTIFICATION_URL is correct
and will work once the notification type is fixed.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

arty.yml

@@ -201,7 +201,7 @@ envs:
     WATCHTOWER_RUN_ONCE: false
     WATCHTOWER_LOG_LEVEL: info
     WATCHTOWER_ROLLING_RESTART: false
-    WATCHTOWER_NOTIFICATIONS: mattermost
+    WATCHTOWER_NOTIFICATIONS: shoutrrr
 
 scripts:
   config: docker compose config