Fix: missing error() helper and broken imports blocking server startup

[Nakshathra-2808]

While testing PR #377, I found the backend wouldn't start at all due to two pre-existing bugs:

1. blueprints/pdf.py was missing required imports (Blueprint, request, error) and pointed to the wrong module for some validators.
2. utils/helpers.py was missing the error() helper function entirely, along with internal imports (send_file, os, jsonify, after_this_request) needed by send_file_and_cleanup.

This fix adds the missing error() function and restores the broken imports so python main.py runs cleanly.

[vercel]

@Nakshathra-2808 is attempting to deploy a commit to the Durgeshwar's projects Team on Vercel.

A member of the Team first needs to authorize it.

[Nakshathra-2808]

Closing this — checked against the latest main and you've already fixed this independently, plus added a success() helper that goes further than my fix. My branch is now outdated/redundant. Thanks for catching it!