Persist map area selection exports

docs/API_CONTRACTS.md

@@ -1013,6 +1013,24 @@ Dataset vector export request:
 }
 ```
 
+Map vector selection export request:
+
+```json
+{
+  "export_kind": "vector_selection",
+  "dataset_id": "uuid",
+  "bbox": {
+    "min_x": 5.0,
+    "min_y": 51.0,
+    "max_x": 5.1,
+    "max_y": 51.1,
+    "crs": "EPSG:4326"
+  },
+  "limit": 250,
+  "name": "optional-basename"
+}
+```
+
 Detection run export request:
 
 ```json
@@ -1050,8 +1068,11 @@ Response persists an `exports` row and writes a deterministic JSON artifact:
 
 Vector dataset exports use the stored dataset GeoJSON. Detection and
 segmentation exports use persisted first-class geometry records and the
-existing Detection/Segmentation GeoJSON conversion services. Raster datasets
-are rejected for dataset GeoJSON export.
+existing Detection/Segmentation GeoJSON conversion services. Vector selection
+exports query persisted PostGIS `vector_features` with the supplied EPSG:4326
+bbox, write the selected FeatureCollection as a `vector_selection_geojson`
+artifact, and persist bbox/feature-count metadata in the export record. Raster
+datasets are rejected for dataset and selection GeoJSON export.
 
 ### POST `/api/v1/exports/metadata`