Remove redundant ways to load environment variables

Which feature or component needs enhancement?
The way we manage environment variables in the project

Describe the current behavior
We seem to have two redundant approaches to loading environment variables from the *.env files.

https://github.com/ProjectTech4DevAI/kaapi-guardrails/blob/1d62fc24821ca6e7b5c7d8aeb473fc9052787a4d/backend/app/core/config.py#L94
and
https://github.com/ProjectTech4DevAI/kaapi-guardrails/blob/1d62fc24821ca6e7b5c7d8aeb473fc9052787a4d/backend/app/load_env.py#L5

Describe the enhancement you'd like
The settings variable exposed in config.py seems to be the one we are relying heavily on throughout the codebase. So we can remove the load_env.py and all calling code referencing that file.

Why is this enhancement needed?
This will prevent confusing overrides of environment variables due to the two approaches interacting in unexpected manner.

Start by comparing backend/app/core/config.py around the settings definition with backend/app/load_env.py, then search the repository for references to load_env.py. Remove the redundant loader and its calling code while retaining the config.py settings path. Done means no remaining references to load_env.py and environment variables use the single intended approach.