Fix Redis rate limiter TTL race condition

[tanishka-kuwar]

🔗 Related Issue

Closes #1759

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📝 Summary of Changes

• Updated the Redis-backed rate limiter to use EXPIRE(..., nx=True) for both minute and hour buckets.
• Prevents resetting the TTL on every request.
• Preserves the original expiration while still restoring it if a key unexpectedly has no TTL.

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🏷️ Type of Change

• 🐛 Bug fix
• ✨ New feature
• ♻️ Refactor
• 📝 Documentation update
• 🎨 UI / Style change
• 🔧 Chore

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🧪 Testing

Ran:

python -m pytest testing/backend/unit -k rate_limiter -v

Result:

• ✅ 17 rate limiter tests passed.

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✅ Checklist

• No merge conflicts
• Changes follow the project guidelines
• Related issue linked
• Changes tested locally

[tanishka-kuwar]

Hi @utksh1,

The backend rate limiter change passes all relevant local tests.

The failing CI appears to be unrelated to this PR. The failure is in the frontend npm audit step due to an existing high-severity vulnerability in the undici dependency:

GHSA-4cwx-7wf7-3272

Since this PR only modifies backend/secuscan/rate_limiter.py, could you please rerun or advise if this dependency issue is expected?

Thank you

[utksh1]

Good fix for Redis TTL race condition using nx=True. Backend-only change; frontend check failures are unrelated.