feat(pdfengines): add flatten form field to split route

pkg/modules/qpdf/doc.go

@@ -3,6 +3,7 @@
 //
 // 1. The merging of PDF files.
 // 2. The splitting of PDF files.
+// 3. Flattening of PDF files
 //
 // The path to the QPDF binary must be specified using the QPDK_BIN_PATH
 // environment variable.

pkg/modules/qpdf/qpdf.go

@@ -101,7 +101,8 @@ func (engine *QPdf) Merge(ctx context.Context, logger *zap.Logger, inputPaths []
 	return fmt.Errorf("merge PDFs with QPDF: %w", err)
 }
 
-// Flatten merges annotation appearances with page content, deleting the original annotations.
+// Flatten merges annotation appearances with page content, deleting the
+// original annotations.
 func (engine *QPdf) Flatten(ctx context.Context, logger *zap.Logger, inputPath string) error {
 	var args []string
 	args = append(args, "--generate-appearances")