fix: add missing Union import in main.py

[devanmolx]

Fixes #187

Problem

Union is used as a type hint in run_pdf_fill_process() but was
never imported, causing an immediate NameError on startup:

NameError: name 'Union' is not defined

Changes

Added to src/main.py:

• from typing import Union

How to verify

1. docker compose up -d
2. docker exec -it fireform-app bash
3. python src/main.py
4. App should run past line 14 without any NameError.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• Self-reviewed my changes
• No new warnings introduced
• Tested locally and verified fix works