Introduces MassImportService.SkipReason with all five values —
INVALID_FILENAME_PATH_TRAVERSAL, INVALID_PDF_SIGNATURE, FILE_READ_ERROR,
ALREADY_EXISTS, S3_UPLOAD_FAILED — making the full set of reasons greppable
and type-safe. SkippedFile.reason changes from String to SkipReason;
importSingleDocument return type updated accordingly. JSON serialisation
is unchanged (Jackson serialises enums by name). All tests updated.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>