test(backend): cover timestamp normalization across timezone offsets

[utksh1]

Summary

Add focused tests for equivalent timestamps using Z and explicit offsets so recency calculations stay stable.

Scope

• Suggested file: backend/secuscan/finding_intelligence.py
• Add or extend the matching backend unit or integration test.
• Keep the change focused on the documented edge case.

Acceptance criteria

• The regression case is reproducible in an automated test.
• The expected safe behavior is asserted explicitly.
• Existing behavior remains covered.
• The focused test passes locally.

Verification

Run the affected pytest file and any directly related suite.

[Sneha-Anand1910]

Hi @utksh1, I'd like to work on this issue under GSSoC '26. Could you please assign it to me?