Fix CI/CD deploy: use stack.env for Portainer-managed stacks

Portainer stores environment variables in stack.env, not .env.
Add --env-file stack.env to compose commands in the deploy step.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/deploy.yml

@@ -48,8 +48,8 @@ jobs:
             -v ${{ vars.DEPLOY_PATH }}:/deploy \
             -w /deploy \
             docker:27-cli sh -c "
-              docker compose -p umbra pull backend frontend &&
-              docker compose -p umbra up -d db backend frontend
+              docker compose -p umbra --env-file stack.env pull backend frontend &&
+              docker compose -p umbra --env-file stack.env up -d db backend frontend
             "
 
       - name: Health check