fix: add csv sanitizer module

[saurabhhhcodes]

Description

Implements server/utils/csvSanitizer.ts so the empty utility file is now a real TypeScript module, then routes CSV export cell serialization through the sanitizer.

Related Issue

Closes #789

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• Test addition or update

Changes Made

• Added sanitizeCsvCell and escapeCsvCell exports for CSV-safe value handling.
• Updated assessmentsToCsv to use the sanitizer for headers and row values.
• Added Vitest coverage for CSV escaping and spreadsheet formula-prefix protection.

Testing

• npm run test -- server/utils/csvExport.test.ts
• git diff --check
• npm run check currently fails on upstream main before this change is reached due syntax errors in server/routes/assessments.routes.ts around lines 118-141.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have added/updated tests where applicable