Title: Add a /healthz endpoint for basic readiness/liveness checks

[nofarblue]

📝 Description
To improve observability and Kubernetes readiness, we should expose a lightweight /healthz endpoint that returns a simple 200 OK status. This helps in:

Readiness/liveness probes in orchestration environments

Health checks for load balancers or monitoring tools

CI tests verifying server availability

✅ Acceptance Criteria
Add a new route /healthz that:

Responds to GET

Returns HTTP 200 with body "OK\n"

Ensure no query parameters are needed

Add unit test to verify:

Status code is 200

Response body is "OK\n"

Add log entry for /healthz requests (optional)

🧩 Notes
This is a simple change that touches routing, handler creation, and test writing. It’s ideal for testing Copilot’s ability to:

Scaffold new routes in a Go web server

Write HTTP tests

Recognize standard health check patterns

No external dependencies are required. Low effort, high visibility.