test(backend): cover rate-limiter behavior across Redis fallback recovery

[utksh1]

Summary

Add a regression test that limits remain conservative and consistent when Redis fails and later recovers.

Scope

• Suggested file: backend/secuscan/rate_limiter.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.