fix: use SELECT 1 for health checks #518

[edehvictor]

Closes #518

---

Refs #18

Changes

• replace the health-check database probe with SELECT 1 through the shared pg pool
• reuse the same database and Horizon probe logic in both �ackend/src/app.js and �ackend/src/index.js
• add focused unit coverage for the new health probe helper

Testing

• npm test -- src/lib/health.test.js
• npm test (currently fails in this branch because of existing missing dependencies/env such as pino, jsonwebtoken, supertest, prom-client, and SUPABASE_URL/SUPABASE_SERVICE_ROLE_KEY)

[vercel]

@edehvictor is attempting to deploy a commit to the Emmanuel's projects Team on Vercel.

A member of the Team first needs to authorize it.