feat: Add MongoDB health monitoring endpoint and harden auth routes with strict rate limiting

[yash1616-accept]

Description

This pull request introduces two crucial production-grade features to enhance the boilerplate's out-of-the-box monitoring and security posture:

1. Database Health Monitor (/v1/health): A dynamic endpoint that checks mongoose.connection.readyState and returns a 200 OK or 503 Degraded status along with system uptime and timestamps.
2. Authentication Rate Limiting: Hardens sensitive routes (/v1/auth/register and /v1/auth/login) against brute-force attacks by restricting rapid consecutive requests using express-rate-limit.

Validation & Testing

• Added comprehensive integration tests in tests/integration/health.test.js.
• Validated all tests using Jest; integration test suites pass successfully.