feat(health): add DB + Horizon readiness checks in /health

[ryzen-xp]

PR Title

feat(health): add DB + Horizon readiness checks in /health

Summary

• /health now includes dependency readiness
• checks:
  • DB: DATABASE_URL, connect/query SELECT 1 (1s timeout)
  • Horizon: HORIZON_URL from listener config, HTTP GET (2s timeout)
• response fields added:
  • ready (true only if both are ok)
  • dependencies.database and dependencies.horizon details
• existing status fields preserved:
  • status: ok
  • service: creditra-backend

Tests

• added coverage in:
  • health.test.ts
  • health.test.ts
  • api.test.ts assertion updated
• run command:
  • npm test (after npm install)
• health tests pass; integration suite has existing unrelated baseline mismatch in other endpoints (not from /health change)

Notes

• no sensitive data introduced

• fast fail vs readiness semantics satisfied

• ready for review/merge with minimal footprint

• Close Health check depth for database and Horizon #101