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>

restic/compose.yaml

@@ -20,7 +20,6 @@ services:
       - backup_directus_uploads:/volumes/directus_uploads:ro
       - backup_directus_bundle:/volumes/directus_bundle:ro
       - backup_awesome_data:/volumes/awesome_data:ro
-      - backup_gotify_data:/volumes/gotify_data:ro
       - backup_scrapyd_data:/volumes/scrapyd_data:ro
       - backup_scrapy_code:/volumes/scrapy_code:ro
       - backup_n8n_data:/volumes/n8n_data:ro
@@ -84,9 +83,6 @@ volumes:
   backup_awesome_data:
     name: awesome_data
     external: true
-  backup_gotify_data:
-    name: messaging_gotify_data
-    external: true
   backup_scrapyd_data:
     name: scrapy_scrapyd_data
     external: true