Add real app health check

[akargi]

Summary

This PR adds a real application-level health check that validates critical dependencies instead of returning a static placeholder response. It checks database connectivity and Redis availability and exposes the result through the main /api/health endpoint.

Related Issues

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that changes existing behaviour)
• Documentation only
• Refactor / chore (no functional change)

Changes

• Added a real health check in app.service.ts that:
  • runs SELECT 1 against the TypeORM data source
  • pings Redis via the shared REDIS_CLIENT
  • returns ok when both dependencies are healthy, otherwise degraded
• Updated the root app controller in app.controller.ts to serve the asynchronous health response at /api/health
• Added regression tests in app.service.spec.ts covering:
  • healthy dependency state
  • degraded dependency state

Checklist

• I have read the CONTRIBUTING guide.
• npm run lint passes.
• npx tsc --noEmit passes (no type errors).
• npm run build succeeds.
• npm test passes, and I added/updated tests for my change where it
  made sense.
• Any Stellar/Soroban network access goes through the stellar module
  services (no direct Horizon/RPC calls from feature modules).
• I did not commit secrets, private keys, or mainnet credentials.

Notes for Reviewers

This is a lightweight app health endpoint intended for load balancers or uptime checks. It is intentionally dependency-aware and returns per-service status for DB and Redis. It does not yet include more advanced checks for queue workers or external providers.