feat(health): implement GET /health/deep readiness probe (#1)

[ghzhost]

Overview

Closes #1

This PR implements the readiness probe endpoint that checks DB connectivity () and Horizon reachability under explicit 2-second timeouts, properly registering the route in and making functional.

Summary of Changes

• Implemented handler in :
  • Runs on with a 2-second timeout.
  • Sends a request to Horizon URL with a 2-second timeout.
  • Returns with when healthy.
  • Returns with and specific failure reasons (, , ) when either check fails.
• Mounted in and declared .