Fix #314: Make Setback Temperature and Setback hours per day truly optional

[chaoyi09]

Summary

• Fix Zod schema to handle empty string form inputs for optional setback fields using z.preprocess
• Make setbackTemperature and setbackHoursPerDay nullable in Prisma schema (Int → Int?)
• Replace hardcoded defaults (|| 65 / || 0) with ?? null in createCase and updateCase
• Handle null values in edit page (show empty field) and analysis display page (show "N/A")
• Allow users to clear previously filled setback values (change from filled → empty)

Migration

• Added database migration to allow NULL values for setback fields

Test

• Leave Setback Temperature and Setback hours per day empty → form submits successfully
• Fill in setback fields → values are saved and displayed correctly
• View case details with empty setback fields → shows "N/A"
• Edit case with empty setback fields → fields remain empty (not pre-filled with defaults)
• Edit case with filled setback fields → user can clear the values and they become "N/A"