feat: add Watchtower for automatic Docker image updates

- Created watch/compose.yaml with Watchtower service configuration
- Added Watchtower environment variables to arty.yml
- Enabled Watchtower monitoring for sexy_frontend container
- Configurable via environment variables (poll interval, cleanup, logging)
- Label-based updates: only containers with watchtower.enable=true

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

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

arty.yml

@@ -56,6 +56,15 @@ envs:
     # Proxy
     PROXY_COMPOSE_PROJECT_NAME: proxy
     PROXY_DOCKER_IMAGE: traefik:latest
+    # Watchtower
+    WATCHTOWER_POLL_INTERVAL: 300
+    WATCHTOWER_LABEL_ENABLE: true
+    WATCHTOWER_CLEANUP: true
+    WATCHTOWER_INCLUDE_STOPPED: false
+    WATCHTOWER_INCLUDE_RESTARTING: true
+    WATCHTOWER_RUN_ONCE: false
+    WATCHTOWER_LOG_LEVEL: info
+    WATCHTOWER_ROLLING_RESTART: false
 
 scripts:
   config: docker compose config