fix: add proper type annotation for threshold parameter in duplicate_service.py

[tmdeveloper007]

Issue Description

The check_duplicate method signature is:
def check_duplicate(self, text: str, threshold: float = None) -> dict:
But None is not a valid float. Should be float | None.

Expected Fix

Change the type annotation to: float | None

Files to Modify

• backend/services/duplicate_service.py (line 196)