Fix deploy workflow YAML for newsletter secret ref

[suguanYang]

Summary

• Fix the deploy workflow YAML introduced in Fix newsletter database secret ref deploy #70 by indenting the newsletter secret patch heredoc inside the run: | block.
• Preserve the optional NEWSLETTER_DATABASE_URL Kubernetes secretKeyRef behavior from Fix newsletter database secret ref deploy #70.
• Addresses failed CD run https://github.com/Ontos-AI/knowhere-dashboard/actions/runs/29238073675, which GitHub reported as a workflow-file issue before jobs started.

Verification

• Parsed .github/workflows/deploy.yml locally with a YAML parser: passed
• git diff --check: passed

Manual browser checks: not applicable, deployment workflow only.

Screenshots or Recordings

Not applicable for non-UI changes.

Deployment Notes

• No behavior change beyond making the workflow parseable again.
• The next merge to staging should re-trigger CD with the optional newsletter secret ref.

Checklist

• Tests were added or updated when behavior changed
• Public docs, examples, or screenshots were updated when needed
• User-visible UI changes were checked on responsive layouts: not applicable, no UI changes
• Auth, billing, and API proxy changes avoid leaking secrets or user data
• The pull request description explains any breaking or user-visible change