Endpoint should return non-polymorphic data types

Currently, it returns `WithId x`. It is not possible to work with this on the frontend. I propose that every response should have its own data type, like `IssuesResponse` which would not have any parameters.

This issue is the blocker for further frontend work at the moment.

Start by locating the endpoint that currently returns `WithId x` and inspect how its response is consumed by the frontend. Determine the response types needed for each endpoint, then verify that the affected frontend work can consume them without polymorphic parameters; the issue provides no file or test names.

Written by the indexing model from the issue text.