Fix tasks API crashes, CSV export, and foreign keys

[PrashantGuptacodecraft]

Related Issue

Closes #ISSUE_NUMBER

Summary

This PR fixes 3 backend bugs: server crashes when adding tasks, broken CSV exports, and missing database constraints.

Changes Made

• Fixed /api/tasks so it no longer crashes the server when tasks have validation errors.
• Added quotes around text fields in the CSV export so commas no longer break the columns.
• Enabled SQLite foreign keys in database.js to protect database relationships.

Testing

• Tested adding multiple invalid tasks; the server returns a 400 error safely instead of crashing.
• Exported a CSV with commas in the text and verified the file opens correctly.
• Verified foreign keys are enabled when the server starts.

Screenshots

N/A

Checklist

• Code follows project style
• Tested locally
• No unrelated changes included
• Documentation updated (if applicable)