Backend: Add end-to-end smoke tests for staging (deploy + basic flows)

[greatest0fallt1me]

Description

Add a minimal e2e smoke test suite that can run against staging to validate critical flows (health, auth, credit list, simulate draw) without requiring chain writes.

Requirements and context

• Must be secure and documented.
• Must avoid destructive operations unless explicitly flagged.

Suggested execution

git checkout -b test/staging-smoke

Implement changes

• Add scripts/smoke/ with environment-driven target URL.
• Add CI job optionally triggered on deploy.

Test and commit

npm test

Example commit message

test(ci): staging smoke tests for critical backend flows

Guidelines

• Timeframe: 96 hours.