feat: cache geojson data for vector files

migrations/00007_add_geojson_to_datasets.sql

@@ -0,0 +1,8 @@
+-- +goose Up
+-- Pre-assembled GeoJSON FeatureCollection served by the /datasets/{id}.geojson
+-- endpoint. Generated at processing time for vector and vector_with_kato
+-- datasets (NULL until generated / for rasters).
+ALTER TABLE datasets ADD COLUMN geojson JSONB;
+
+-- +goose Down
+ALTER TABLE datasets DROP COLUMN geojson;