n+1 database queries when fetching /content_nodes and /camps endpoints

There is an n+1 problem in the API (this one has nothing to do with the frontend).
When fetching the /content_nodes endpoint, there is a separate SQL statement executed for each material node (loading the connected material items) and for each checklist node (loading the connected checklist items).
When fetching the /camps endpoint, there are 3 separate SQL statements executed for each camp, loading the periods, camp collaborations and material lists.

This can be reproduced locally by visiting http://localhost:3000/api/content_nodes or http://localhost:3000/api/camps and afterwards http://localhost:3000/api/_profiler/empty/search/results?limit=10, clicking on the topmost Token and then on the Doctrine section in the left navigation.

It originally became evident during #8004, when new content node fixtures were added and the query counts in the EndpointPerformanceTest (for [content nodes](https://github.com/ecamp/ecamp3/pull/8004#discussion_r2320882992) and for [camps](https://github.com/ecamp/ecamp3/pull/8004#discussion_r2320888044)) went up.

This could potentially be solved by using Doctrine's `fetch: EAGER` feature. According to the [documentation](https://www.doctrine-project.org/projects/doctrine-orm/en/3.5/reference/working-with-objects.html#by-eager-loading) and some rudimentary tests, for one-to-many relations, doctrine can query the top-level entity and then query the related entities in a second query (or in batches of 100 by default, if there are many). But eager fetching like this can potentially cause performance impacts in other endpoints.