Request: expose DPI to image scale constant of `72`

### Requested feature

It seems sprinkled across the Docling codebase is a magic number 72. For example, .

https://docling-project.github.io/docling/examples/export_multimodal/ mentions:

> `IMAGE_RESOLUTION_SCALE`: page rendering scale (1 ~ 72 DPI).

In my own code, I work with DPI. So I need to convert DPI to `image_scale` for `PdfPipelineOptions`. Can Docling expose this as an importable constant?

```python
IMAGES_SCALE_PER_DPI: int = 72

dpi = 144
print(dpi / IMAGES_SCALE_PER_DPI) # 2
```

### Alternatives

Redefining the conversion constant 72 in my own code, which is the workaround.