feat(api): add system resource monitoring functionality

- implement getSystemInfo to gather CPU and memory usage
- add platform-specific implementations for memory and CPU usage
- enhance OpenAPI documentation to include system resource metrics

internal/api/api.go

@@ -239,6 +239,7 @@ func apiHandler(w http.ResponseWriter, r *http.Request) {
 	mu.Lock()
 	app.Info["host"] = r.Host
 	app.Info["pid"] = os.Getpid()
+	app.Info["system"] = getSystemInfo()
 	mu.Unlock()
 
 	ResponseJSON(w, app.Info)