bug: Zenodo parser crashes with isoformat AttributeError

## Bug

Searching with Zenodo as a source produces:

```
[WARN] zenodo: 'str' object has no attribute 'isoformat'
```

## Steps to reproduce

```bash
paper-search search "machine learning" --sources zenodo --max-results 5
```

## Expected

Date fields that are already strings should be handled without calling `.isoformat()` on them.