feat(datasets): periods (start_date/end_date), dg_id column, date filters; rename observation dates

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/repository/postgres/district.go

@@ -9,14 +9,14 @@ import (
 )
 
 // ListAllObservations returns every observation for a dataset ordered by
-// (kato_code, date). Unlike ListObservations it is unpaginated: it backs GeoJSON
+// (kato_code, start_date). Unlike ListObservations it is unpaginated: it backs GeoJSON
 // assembly, where all of a dataset's observations are needed at once.
 func (r *DatasetRepository) ListAllObservations(ctx context.Context, datasetID uuid.UUID) ([]domain.Observation, error) {
 	rows, err := r.pool.Query(ctx,
 		`SELECT `+observationColumns+`
 		 FROM dataset_observations
 		 WHERE dataset_id = $1
-		 ORDER BY kato_code, date`,
+		 ORDER BY kato_code, start_date`,
 		datasetID)
 	if err != nil {
 		return nil, mapError(err)