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>

arty.yml

@@ -48,11 +48,6 @@ envs:
     SEXY_PASSWORD_RESET_URL_ALLOW_LIST: https://sexy.pivoine.art/password/reset
     SEXY_FRONTEND_PUBLIC_API_URL: https://sexy.pivoine.art/api
     SEXY_FRONTEND_PUBLIC_URL: https://sexy.pivoine.art
-    # Gotify
-    GOTIFY_TRAEFIK_ENABLED: true
-    GOTIFY_COMPOSE_PROJECT_NAME: messaging
-    GOTIFY_IMAGE: gotify/server:latest
-    GOTIFY_TRAEFIK_HOST: gotify.pivoine.art
     # Mattermost
     MATTERMOST_TRAEFIK_ENABLED: true
     MATTERMOST_COMPOSE_PROJECT_NAME: mattermost