BUG: Expand API endpoint does not include referencing CallForTenders entities

[tillprochaska]

Describe the bug
When viewing a LegalEntity that is referenced by a CallForTenders in the Aleph UI, it doesn’t show a tab that links to the referencing CfT.

For other entity types Aleph displays a link back to the referencing entity. In the following example a Company entity references an Address entity via the addressEntity property.

To Reproduce
Steps to reproduce the behavior:

1. Create an investigation
2. In the table editor, create a Company entity
3. In the table editor, create a CallForTenders entity and add the Company entity to the tenderers property.
4. Navigate to the Company entity (for example by clicking on the "Expand" icon button next to the row in the table editor).
5. There’s no link back to the CfT.

Expected behavior
There should be a tab named "Tenders" when viewing the Company entity.

(I’m not entirely sure though if the fact that it’s missing here isn’t due to a UX or technical reason, so it’s worth checking the history of the current implementation.)

Aleph version
3.15.5

Additional context
This is an issue with the /entities/<id>/expand endpoint of the Aleph API. I have verified that in this case the response for this endpoint doesn’t include CfTs.