Added api/classifications endpoint, requiring a new query

internal/store/store.go

@@ -717,6 +717,21 @@ func (s *Store) ListReleases(
 	return releases, nil
 }
 
+// ListClassifications returns every classification code that at least one
+// stored opportunity has, ordered by code. A code no opportunity has any more
+// isn't listed. The result is never nil.
+func (s *Store) ListClassifications(
+	ctx context.Context,
+) ([]types.ClassificationCode, error) {
+	classifications := []types.ClassificationCode{}
+	err := s.db.SelectContext(ctx, &classifications, selectClassificationsInUse)
+	if err != nil {
+		return nil, fmt.Errorf("listing classifications: %w", err)
+	}
+
+	return classifications, nil
+}
+
 // LatestReleaseDate returns the date of the newest release stored for the
 // opportunities from source, or the zero time if none are stored, so a
 // fetcher can carry on from where it left off.