- Added is_yearly_data_complete() function to check if yearly CSV files are complete
- Complete data criteria: file ends with 31.12.YYYY and starts with valid first working day
- Prevent re-downloading of complete yearly data files
- Improved efficiency by avoiding unnecessary downloads of multi-megabyte files
- Added proper CSV parsing for completeness checking
- Updated data_fetcher.py with intelligent download logic
- Maintains backward compatibility with force download option