[Bug] CSV export fails with 500 Internal Server Error for group names with special characters

[Uli-Z]

Bug Description

When attempting to export expenses as a CSV file from a group that has a name containing special characters (e.g., German umlauts like ä, ö, ü), the server returns a 500 Internal Server Error.

Steps to Reproduce

1. Create a group with a name containing special characters (e.g., "Büroausgaben").
2. Add some expenses to the group.
3. Go to the group's expenses page and click on "Export as CSV".
4. Observe the network request and server response.

Expected Behavior

The server should successfully generate and return a CSV file with a correctly encoded filename that includes the special characters. For example, Spliit Export - Büroausgaben - 2025-11-11.csv.

Actual Behavior

The server responds with a 500 Internal Server Error, and the CSV file is not downloaded.