fix: use env_file for L2 server config

- Add env_file support for L2_SERVER and L2_DOMAIN
- Fix typo in L2_SERVER URL (ros-ash -> rose-ash)
- Add .env.example with required variables

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

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

docker-compose.yml

@@ -14,10 +14,12 @@ services:
 
   l1-server:
     image: git.rose-ash.com/art-dag/l1-server:latest
+    env_file:
+      - .env
     environment:
       - REDIS_URL=redis://redis:6379/5
       - CACHE_DIR=/data/cache
-      - L2_SERVER=http://activitypub_l2-server:8200
+      # L2_SERVER and L2_DOMAIN from .env file
     volumes:
       - l1_cache:/data/cache
     depends_on: