feat: cache geojson data for vector files

internal/transport/http/dataset_handler.go

@@ -304,11 +304,11 @@ func (h *DatasetHandler) get(w http.ResponseWriter, r *http.Request) {
 	httputil.WriteJSON(w, http.StatusOK, dataset)
 }
 
-// geojson returns the dataset as a GeoJSON FeatureCollection (RFC 7946). A
-// vector_with_kato dataset always ignores its own geometry and instead joins the
-// districts table on KATO code: one feature per KATO with the observation values
-// mapped onto its district polygon. A plain vector dataset serves its own
-// geometry as a single feature.
+// geojson returns the dataset's pre-assembled GeoJSON FeatureCollection (RFC
+// 7946), generated and stored at processing time. A vector_with_kato dataset's
+// collection joins the districts table on KATO code (one feature per KATO with
+// the observation values mapped onto its district polygon); a plain vector
+// dataset's collection wraps its own geometry as a single feature.
 func (h *DatasetHandler) geojson(w http.ResponseWriter, r *http.Request) {
 	id, ok := parseUUIDParam(w, r, "id")
 	if !ok {
@@ -321,7 +321,7 @@ func (h *DatasetHandler) geojson(w http.ResponseWriter, r *http.Request) {
 	}
 	w.Header().Set("Content-Type", "application/geo+json")
 	w.WriteHeader(http.StatusOK)
-	_ = json.NewEncoder(w).Encode(fc)
+	_, _ = w.Write(fc)
 }
 
 func (h *DatasetHandler) download(w http.ResponseWriter, r *http.Request) {