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>

core/postgres/init/01-init-databases.sh

@@ -29,17 +29,22 @@ psql -v ON_ERROR_STOP=1 --username "$POSTGRES_USER" --dbname "$POSTGRES_DB" <<-E
     SELECT 'CREATE DATABASE joplin'
     WHERE NOT EXISTS (SELECT FROM pg_database WHERE datname = 'joplin')\gexec
 
+    -- Mattermost chat platform database
+    SELECT 'CREATE DATABASE mattermost'
+    WHERE NOT EXISTS (SELECT FROM pg_database WHERE datname = 'mattermost')\gexec
+
     -- Grant privileges to all databases
     GRANT ALL PRIVILEGES ON DATABASE directus TO $POSTGRES_USER;
     GRANT ALL PRIVILEGES ON DATABASE umami TO $POSTGRES_USER;
     GRANT ALL PRIVILEGES ON DATABASE n8n TO $POSTGRES_USER;
     GRANT ALL PRIVILEGES ON DATABASE linkwarden TO $POSTGRES_USER;
     GRANT ALL PRIVILEGES ON DATABASE joplin TO $POSTGRES_USER;
+    GRANT ALL PRIVILEGES ON DATABASE mattermost TO $POSTGRES_USER;
 
     -- Log success
     SELECT 'Compose databases initialized:' AS status;
     SELECT datname FROM pg_database
-    WHERE datname IN ('directus', 'umami', 'n8n', 'linkwarden', 'joplin')
+    WHERE datname IN ('directus', 'umami', 'n8n', 'linkwarden', 'joplin', 'mattermost')
     ORDER BY datname;
 EOSQL
 
@@ -53,4 +58,5 @@ echo "  • umami        - Tracking database"
 echo "  • n8n          - Workflow automation database"
 echo "  • linkwarden   - Bookmark manager database"
 echo "  • joplin       - Note-taking server database"
+echo "  • mattermost   - Chat platform database"
 echo ""