feat: warn on missing startup env vars

[saurabhhhcodes]

What changed

• Added a small startup helper that warns when essential runtime env vars are missing.
• Hooked the check into both WSGI and ASGI startup paths.
• Added a test that verifies the warning is printed for missing Gemini/Google OAuth settings.

Why

• The app could start successfully and only fail later when a feature needed an unset credential.
• A clear startup warning makes misconfiguration easier to catch before deployment.

Validation

• python3 -m py_compile backend/backend/startup.py backend/backend/wsgi.py backend/backend/asgi.py backend/users/tests.py
• git diff --check
• python3 backend/manage.py test users.tests.StartupValidationTests -v 2

Closes #405

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