feat: add health check handler

http/public.go

@@ -136,3 +136,8 @@ func authenticateShareRequest(r *http.Request, l *share.Link) (int, error) {
 
 	return 0, nil
 }
+
+func healthHandler(w http.ResponseWriter, _ *http.Request) {
+	w.WriteHeader(http.StatusOK)
+	_, _ = w.Write([]byte(`{"status":"OK"}`))
+}