feat: add startup settings warnings

[saurabhhhcodes]

Adds startup-time warnings for missing critical environment settings so misconfigured deployments fail visibly instead of breaking later in feature code.

What changed:

• added a small startup helper that prints a bold-style warning listing missing critical settings
• invoked the helper from both wsgi.py and asgi.py after Django setup
• added unit tests for both the warning and quiet paths

Validation:

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

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