feat: configure Mattermost webhook integrations

Added Mattermost webhook notifications for infrastructure services:

- **Netdata**: Health monitoring alerts sent to Mattermost
  - Configured via MATTERMOST_WEBHOOK_URL environment variable
  - All alert roles route to Mattermost notifications channel

- **Restic/Backrest**: Backup status notifications
  - Webhook URL passed as environment variable
  - Backrest web UI can be configured to use the webhook

- **Watchtower**: Container update notifications
  - Uses Shoutrrr mattermost:// URL format
  - Sends notifications when containers are updated

Configuration:
- Webhook URL stored securely in .env file
- All services reference ${MATTERMOST_WEBHOOK_URL} variable
- Watchtower uses specialized Shoutrrr format URL

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

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

arty.yml

@@ -163,6 +163,7 @@ envs:
     WATCHTOWER_RUN_ONCE: false
     WATCHTOWER_LOG_LEVEL: info
     WATCHTOWER_ROLLING_RESTART: false
+    WATCHTOWER_NOTIFICATIONS: mattermost
 
 scripts:
   config: docker compose config