feat: cache geojson data for vector files

internal/domain/dataset.go

@@ -217,6 +217,11 @@ type Dataset struct {
 	// Geometry is the dataset's spatial geometry, serialized as GeoJSON.
 	// Nullable; populated from the file's spatial data.
 	Geometry json.RawMessage `json:"geometry"`
+	// GeoJSON is the pre-assembled GeoJSON FeatureCollection served by the
+	// .geojson endpoint. It is generated and persisted at processing time for
+	// vector and vector_with_kato datasets. Nullable; not exposed on the dataset
+	// response (served only by the .geojson endpoint).
+	GeoJSON json.RawMessage `json:"-"`
 	// BBox is the axis-aligned bounding box [minX, minY, maxX, maxY] derived
 	// from the geometry. Included in responses only for raster datasets.
 	BBox      []float64 `json:"bbox,omitempty"`