feat(front): add members download button

[loadez]

📝 Description

• Add members download button on people page
• Implement export members as csv endpoint
• Add export members test

✅ Checklist

• I have tested this change
• This change requires documentation update

[Copilot]

Pull Request Overview

This PR adds functionality to export organization members as a CSV file. The changes introduce a download button on the people page that allows users to export member data (name, email, and GitHub login) in CSV format.

Key changes:

• Added a new /people/export endpoint that generates and downloads a CSV file containing organization members
• Added a download button in the members list UI (shown only when viewing organization scope)
• Added basic test coverage for the export endpoint

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
front/lib/front_web/controllers/people_controller.ex	Implements organization_users action to fetch members and generate CSV export
front/lib/front_web/router.ex	Adds GET route /people/export for the new export functionality
front/lib/front_web/templates/people/members/members_list.html.eex	Adds download button in UI that links to the export endpoint
front/test/front_web/controllers/people_controller_test.exs	Adds test case verifying 404 response when user lacks organization access

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