Feat : add HealthController with tests and update documentation for Java 21

[Sahilmansoori373]

Summary

This PR introduces a new /api/health endpoint along with its unit tests, and updates project documentation for Java 21 compatibility.

What’s Included

Backend

• Added HealthController to expose a simple health-check API
  • Returns HTTP 200 OK with JSON: { "status": "ok" }
• Added HealthControllerTests using @WebMvcTest
  • Verifies endpoint responds correctly
  • Mock security context included

Documentation

Updated references to Java 21 and project setup in:

• README.md
• CONTRIBUTING.md
• INSTALL.md
• Additional formatting and clarity improvements

Why These Changes Matter

• Enhances readiness for deployment environments (Kubernetes liveness/readiness probes)
• Improves test coverage and reliability for core API health
• Ensures documentation aligns with the updated Java version to help new contributors

Validation

• All tests passing using: ./mvnw test
• Verified API manually using curl/browser

Ready for Review

No production breaking changes. Safe to merge.