Establish a coverage threshold in CI for backend

[mercy60]

Internal ID: #88

Imported from ISSUES.md via scripts/import-issues.mjs.

Problem Statement. No coverage tool is wired. Today's spec files are smoke tests.

Expected Outcome.

• jest --coverage with a threshold statement (coverageThreshold in package.json).
• CI fails when coverage drops.

Acceptance Criteria.

• A simple roundtrip-service test brings coverage above the threshold.

Implementation Notes.

• Start with 60%; iterate up.

Files / modules affected.

• backend/package.json
• .github/workflows/build.yml (add Node CI)

Dependencies. None.

Difficulty / Effort. Medium / M (≈ 1 day).

Labels. testing, area:backend, ci