fix: add range validation for confidence_threshold and duplicate_sensitivity in TicketRequest

[tmdeveloper007]

Issue Description

The TicketRequest Pydantic model in backend/main.py has confidence_threshold (default 0.20) and duplicate_sensitivity (default 0.85) fields with no range validation. Users could submit negative values or values > 1.0.

Expected Fix

Add @field_validator decorators to validate that confidence_threshold and duplicate_sensitivity are within [0.0, 1.0] range.

Files to Modify

• backend/main.py (lines 361-383)