Fix nominee visibility during open elections

[Parzival235]

Summary

• scope the open-election nominee list to the election in the URL
• show both candidates nominated by the signed-in user and the user's own candidacy
• allow nominators to open nominee detail pages during the nomination window
• add regression coverage for election scoping and nominee-detail permissions

Root cause

The authenticated-user branch in NomineeList.get_queryset() filtered only on Nominee.user and did not filter by election. This returned nominations of the user across election cycles instead of nominations the user had made in the current election. Separately, Nominee.visible() did not consider the nominator, so a candidate linked from the corrected list still returned 404.

Validation

• DATABASE_URL=sqlite:///:memory: uv run --group dev python manage.py test apps.nominations.tests — 66 tests passed
• ruff check apps/nominations/models.py apps/nominations/views.py apps/nominations/tests/test_views.py
• ruff format --check apps/nominations/models.py apps/nominations/views.py apps/nominations/tests/test_views.py

Fixes #3094