perf(groups): Optimize prefetch queries to fetch only required fields

[joaocfernandes]

Details

Fetch only fields needed by PartialUserSerializer, avoiding unnecessary column fetching

In a local example this reduces database load and object instantiation time by ~30%.

Combined with #18446 , for huge user numbers in a local setup we were able to have significant performance gains, from around 1.3s to about 600ms.

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Checklist

• Local tests pass (ak test authentik/)
• The code has been formatted (make lint-fix)

If an API change has been made

• The API schema has been updated (make gen-build)

If changes to the frontend have been made

• The code has been formatted (make web)

If applicable

• The documentation has been updated
• The documentation has been formatted (make docs)

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