Add map area selection extraction

backend/README.md

@@ -574,6 +574,14 @@ history, known limitations and print-friendly CSS.
 This remains a simple HTML export. It does not add a PDF designer, report
 builder, live provider fetching or new analysis behavior.
 
+## Vector area selection
+
+`POST /api/v1/projects/{project_id}/datasets/{dataset_id}/vector/select` runs a
+read-only EPSG:4326 bbox query against persisted PostGIS `vector_features` and
+returns a canonical-envelope GeoJSON FeatureCollection. It is intended for the
+Map workspace area-extract flow and does not create derived datasets or export
+records.
+
 ## Helpful repository scripts
 
 - `bash scripts/backend_install.sh`