[praseodym]

What & why

When an investigation with corrected results was deleted, its data entry stayed behind. Adding a new investigation for the same polling station and concluding it then gave an internal server error. This is fixed by deleting any linked data entry when an investigation is deleted. Fixes #4172.

How to test

1. Log in as coördinator GSB 1.
2. Go to Corrigendum Gemeenteraad Juinen 2026 (GSB CSO) -> Aangevraagde onderzoeken.
3. Delete the investigation for "Test ander gebouw".
4. Create a new investigation for "Test ander gebouw" and save it.

This works with this PR. On main it gives an internal server error.

Reviewer notes

• The first commit adds tests (they fail on main), the second commit fixes the issue and makes the tests pass.
• delete_investigation_for_polling_station deletes any linked data entry as well. Both the investigation delete endpoint and the polling station delete endpoint use it.
• investigation_repo::delete returns a DataEntryAlreadyLinked error if the polling station still has a data entry, to prevent this issue from reoccurring in another code path.
• Later: move the delete helpers from api/ to service/.

[jschuurk-kr]

In case someone else goes down the same road: I was expecting the fix to include a modal about removing the data entry. It doesn't include it because the modal already exists:

[oliver3]

No comments that are worth another review cycle

[jschuurk-kr]

Tested a variety of scenarios of deleting investigations, data entries and polling stations. Did not find any issues causing incorrect data states.

Did discover it's a good idea to fetch data entry status more often on the investigation pages. Created PR #4218 for that.