Add database connection health check with latency

[D-Bluemoon]

Summary

This PR updates PostgreSQL health checks to report real round-trip latency and server time, and it degrades overall service health when DB latency exceeds 1000ms.

What changed

• health.ts
  • checkPostgresql() now measures latency with performance.now()
  • query result extraction now reads serverVersion from SELECT 1, NOW() AS "serverVersion"
  • returns { healthy, latencyMs, serverVersion }
  • health status is marked degraded when PostgreSQL is connected but latency > 1000ms
• schemas.ts
  • added optional healthy and serverVersion fields to DependencyHealth
• health.test.ts
  • added tests for normal DB latency and slow-query degraded behavior
• health.ts
• index.ts
• index.ts
  • updated service health DB checks to use SELECT 1, NOW() AS "serverVersion" so the shared helper can capture server time and latency

Rationale

• SELECT 1 only verified connectivity, not latency.
• performance.now() provides a more accurate in-process RTT measurement than Date.now().
• Adding healthy and serverVersion makes the DB dependency health richer and more actionable.
• Degrading overall health on high latency catches slow DB responses even when the DB is still reachable.

Fixes / Closes

• Fixes / Closes: Add database connection health check with latency #364

Type of Change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature
• Breaking Change
• Refactoring / Performance
• Documentation / CI

Verification & Test Plan

Automated Tests

pnpm --filter @bettapay/validation test

Manual Verification

1. Call the health endpoint for API Gateway / Indexer / Settlement Engine.
2. Confirm the postgresql dependency now includes latencyMs, healthy, and serverVersion.
3. Simulate a slow DB response and verify the overall health becomes degraded when latency exceeds 1000ms.

Backend Services (BettaPay-Backend)

• Prisma models updated and migrated (if database schema changes) — N/A
• Zod validation checks added/updated
• API routes and services build successfully (pnpm build)
• Local env variables updated in .env.example — N/A

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation — N/A
• My changes generate no new warnings

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