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

Move DATABASE_URL and POSTGRES_PASSWORD to .env via env_file.
docker stack deploy no longer needs env vars sourced, and
repeat deploys won't trigger spurious restarts.

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/activity-pub
-            set -a && source .env && set +a
             docker stack deploy -c docker-compose.yml activitypub
             echo 'Waiting for services to update...'
             sleep 10