fix: prevent non-admin users changing another user's gitAccount

[andypols]

Summary

A recent penetration test identified a security issue in the /api/v1/auth/gitAccount endpoint:

• Users can add or update their own GitHub/GitLab account names.
• Admins can add or update GitHub/GitLab account names for any user.
• However, the API does not enforce this restriction — any user can change the Git account name for any other user, regardless of their permissions.

Root Cause

The API only checks that the requester is authenticated; it does not verify whether the user is updating their own account or has admin privileges.

Fix

This PR introduces a guard clause that ensures the requesting user is either:

• Updating their own Git account
• Has admin privileges.

This change brings the backend access control in line with the intended behaviour already enforced in the UI.

I’ve added a new auth.test.js. I chose not to expand the existing testLogin.test.js because I wanted to test at a lower level, where I can directly assert that the database is updated as expected.

While the current endpoint lives under /auth, updating a user's Git account may conceptually belong on the /user endpoint. However, this change is out of scope for this fix and has been deferred.

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[jescalada]

LGTM! 👍🏼

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.41%. Comparing base (9560617) to head (d407b3b).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1093      +/-   ##
==========================================
+ Coverage   77.37%   77.41%   +0.03%     
==========================================
  Files          55       55              
  Lines        2272     2276       +4     
  Branches      255      255              
==========================================
+ Hits         1758     1762       +4     
  Misses        484      484              
  Partials       30       30              

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