Eliminate ${VAR} substitutions from docker-compose.yml

Move DATABASE_URL, ADMIN_TOKEN, ARTDAG_CLUSTER_KEY to .env
and use env_file on all services. This means docker stack deploy
no longer needs env vars sourced in the shell, and repeat deploys
won't trigger spurious restarts on unchanged services.

GPU worker gets its own .env.gpu with fully resolved cross-VPC URLs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -55,7 +55,6 @@ jobs:
         run: |
           ssh "root@$DEPLOY_HOST" "
             cd /root/art-dag/celery
-            set -a && source .env && set +a
             docker stack deploy -c docker-compose.yml celery
             echo 'Waiting for services to update...'
             sleep 10