feat(http): Add health check endpoint

Add /_/healthz endpoint that returns 200 OK for service health monitoring.

app/main.go

@@ -19,6 +19,11 @@ type Config struct {
 func main() {
 	config := loadConfig()
 
+	http.HandleFunc("/_/healthz", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte("OK"))
+	})
+
 	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
 		proxyHandler(w, r, config)
 	})