feat: add robust validation and 100% branch coverage for _derive_grade

[BHOGALA-SRIKA]

Closes #10

Description

This PR resolves the issue tracking critical business logic testing for the backend grading utility. It introduces comprehensive validation checking and branch coverage for _derive_grade in main.py to prevent regression bugs.

Changes Made

• Type Validation: Added explicit type checks using isinstance to catch and gracefully reject non-numeric inputs (str, NoneType, list, bool), raising a clean ValueError instead of unhandled runtime crashes.
• Scale Boundaries: Implemented strict value limits ensuring inputs reside safely between 0 and 100 inclusive.
• Test Isolation Mocks: Configured sys.modules interception in tests/test_ci.py to isolate tests from heavy third-party framework layers (torch binary paths and supabase network connections), dropping local CI suite test execution to < 2 seconds.

Checklist

• npm run lint passes with no errors
• npm run build compiles without TypeScript errors
• python -m pytest passes (including new tests I added)
• No .env files, API keys, secrets, model weights, or __pycache__ in this diff
• Branch is rebased on main, not merged

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