[Bug] backupCase uses hardcoded path instead of Config.DATA_STORAGE

[lil-aditya]

Summary

The backupCase endpoint in UploadRoute.py uses a hardcoded filesystem path:

Path('WebAPP', 'DataStorage', case)

### Expected behavior


The `backupCase` endpoint should:

1. Use the configured storage directory defined by `Config.DATA_STORAGE`.
2. Work correctly regardless of where the data storage directory is located.
3. Create ZIP archives with relative paths so they can be extracted cleanly on other systems.

### Reproduction steps

1. Configure `DATA_STORAGE` in `Config.py` to a custom directory (e.g. `/mnt/storage`).
2. Start the server.
3. Call the backup endpoint:


### Environment

OS: Windows 11  
Python: 3.x  
Project branch: main  
Server: Flask development server

### Logs or screenshots

_No response_

[lil-aditya]

Note: The hardcoded path issue was recently addressed upstream using
Config.validate_path(Config.DATA_STORAGE, ...).

This PR focuses on the remaining issue where ZIP archives store
absolute filesystem paths instead of relative paths.