Add admin CSV uploader for alumni imports

[mario-ah-salamanca]

Summary

• Adds an admin-visible Board tools link for importing alumni CSV files
• Reuses the existing import_csv command logic for validation and importing
• Validates the uploaded CSV first; if there are bad rows, no users are imported and an import_errors.csv file is returned
• Updates frontend setup to Node 24 LTS + Corepack Yarn so local builds generate webpack-stats.json reliably

Testing

• .venv/bin/python manage.py check
• .venv/bin/python manage.py test registry.tests.test_019_import_csv
• corepack yarn install --frozen-lockfile
• corepack yarn build

Notes for Leo / board review

This is intentionally small and admin-only. Please check whether the CSV flow is acceptable for board members on the portal, then merge to main and test on live/staging before production.

I did not include the open Dependabot updates here; those should stay separate dependency-maintenance PRs.