feat: remove Gotify notification service

Removed Gotify completely from infrastructure, replaced by Mattermost:

- Removed gotify/compose.yaml stack
- Removed Gotify environment variables from arty.yml
- Removed Gotify from compose.yaml include list
- Removed Gotify volume backup from Restic configuration

Gotify has been fully replaced by Mattermost for:
- Infrastructure notifications (Netdata, Watchtower, Restic)
- n8n workflow notifications
- Team collaboration and chat

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

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

compose.yaml

@@ -3,7 +3,6 @@ include:
   - core/compose.yaml
   - awsm/compose.yaml
   - sexy/compose.yaml
-  - gotify/compose.yaml
   - mattermost/compose.yaml
   - scrapy/compose.yaml
   - n8n/compose.yaml