fix: add Float import to business_day model (Phase 2E crash fix)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

local_backend/models/business_day.py

@@ -1,4 +1,4 @@
-from sqlalchemy import Column, Integer, String, DateTime, ForeignKey, Text
+from sqlalchemy import Column, Integer, String, DateTime, Float, ForeignKey, Text
 from sqlalchemy.orm import relationship
 from datetime import datetime, timezone
 from database import Base