feat: Districts table and geojson stub

internal/domain/geojson.go

@@ -0,0 +1,31 @@
+package domain
+
+import "encoding/json"
+
+// District is an administrative district boundary keyed by KATO code. Geometry
+// is the boundary serialized as a GeoJSON geometry object in EPSG:4326.
+type District struct {
+	Kato     string          `json:"kato"`
+	Name     string          `json:"name"`
+	Geometry json.RawMessage `json:"geometry"`
+}
+
+// GeoJSON object type tags (RFC 7946).
+const (
+	GeoJSONFeatureCollection = "FeatureCollection"
+	GeoJSONFeature           = "Feature"
+)
+
+// FeatureCollection is a GeoJSON FeatureCollection (RFC 7946).
+type FeatureCollection struct {
+	Type     string    `json:"type"`
+	Features []Feature `json:"features"`
+}
+
+// Feature is a GeoJSON Feature (RFC 7946). Geometry is a raw GeoJSON geometry
+// object (or JSON null when absent); Properties is an arbitrary key/value map.
+type Feature struct {
+	Type       string          `json:"type"`
+	Geometry   json.RawMessage `json:"geometry"`
+	Properties map[string]any  `json:"properties"`
+}