fix: use env_file for all server config

- Move ARTDAG_DOMAIN, ARTDAG_USER, ARTDAG_L1, JWT_SECRET to .env
- Update docker-stack.yml L1 config to use env_file
- Add .env.example with all required variables

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker-compose.yml

@@ -6,10 +6,8 @@ services:
     env_file:
       - .env
     environment:
-      - ARTDAG_DOMAIN=artdag.rose-ash.com
-      - ARTDAG_USER=giles
       - ARTDAG_DATA=/data/l2
-      - ARTDAG_L1=http://celery_l1-server:8100
+      # ARTDAG_DOMAIN, ARTDAG_USER, ARTDAG_L1, JWT_SECRET from .env file
     volumes:
       - l2_data:/data/l2
     networks: