fixed Abstraction violation (#1169)

[markadodo]

Description

This PR fixes an abstraction violation in XP calculation.

Problem: The previous code first queries the database in Elixir to get team and submission IDs, then it queries the submission table again using those IDs. This mixes app-level logic with DB logic, breaking the abstraction.

Solution: Do all filtering and joining directly in the database using Ecto joins.

Fixes: #1169

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Code quality improvements

How to test

Run

mix test test/cadet/assessments/assessments_test.exs

in the terminal

alternatively:

1. Change the XP of a submission of a student
2. Check if it has been reflected correctly

Checklist

• I have tested this code
• I have updated the documentation