fix: reduce default dpi to 350 (#4199)
<!-- CURSOR_SUMMARY -->
> [!NOTE]
> **Enhancement**
>
> - Change default `PDF_RENDER_DPI` from `400` to `350` in
`partition/utils/config.py`
>
> **Project updates**
>
> - Bump version to `0.18.31-dev0`
> - Update `CHANGELOG.md` to note DPI change
>
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
aa29ade193ff153b5461faf7c112b59d97108d34. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->
