Add /healthz endpoint for basic readiness/liveness checks

[Copilot]

Adds a lightweight /healthz endpoint to improve observability and support Kubernetes readiness/liveness probes.

Changes Made

• New endpoint: Added /healthz route that responds to GET requests
• Response format: Returns HTTP 200 status with body "OK\n"
• Logging: Added optional logging for health check requests
• Testing: Added HTTP integration test to verify status code and response body
• Build: Updated .gitignore to exclude binary files

Implementation Details

func healthzHandler(w http.ResponseWriter, r *http.Request) {
    log.Printf("Health check request received")
    w.WriteHeader(http.StatusOK)
    w.Write([]byte("OK\n"))
}

The endpoint follows standard health check patterns and requires no query parameters. It's designed for:

• Kubernetes readiness/liveness probes
• Load balancer health checks
• CI tests verifying server availability

Testing

curl -i http://localhost:8080/healthz
# HTTP/1.1 200 OK
# Content-Length: 3
# 
# OK

All existing functionality remains unchanged and tests continue to pass.

Fixes #7.

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