feat: add Mattermost team chat platform to replace Gotify

Added Mattermost stack to the docker-compose infrastructure:

- **Mattermost stack** (mattermost.pivoine.art):
  - Team collaboration and chat platform
  - PostgreSQL backend for message persistence
  - Email notifications via IONOS SMTP
  - Support for channels, direct messages, and integrations
  - Mobile and desktop app support
  - Full Traefik integration with SSL termination

Infrastructure updates:
- Updated PostgreSQL init script to create mattermost database
- Added environment variables to arty.yml for Mattermost configuration
- Updated compose.yaml include list
- Configured email settings for notifications and invitations

This will replace Gotify as the primary notification and messaging platform.
All services integrated with Traefik for SSL termination and include
Watchtower auto-update labels.

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

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

arty.yml

@@ -53,6 +53,12 @@ envs:
     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
+    MATTERMOST_IMAGE: mattermost/mattermost-team-edition:latest
+    MATTERMOST_TRAEFIK_HOST: mattermost.pivoine.art
+    MATTERMOST_DB_NAME: mattermost
     # Scrapy
     SCRAPY_TRAEFIK_ENABLED: true
     SCRAPY_COMPOSE_PROJECT_NAME: scrapy