refactor: asyncio for better cancellation handling

[sghng]

With asyncio, we're now able to terminate subprocesses gracefully whenever an
error is first encountered in any branch.

[Copilot]

Pull Request Overview

This PR refactors the codebase to use asyncio for improved error handling and graceful subprocess cancellation. When an error occurs in any parallel branch of the conversion pipeline, all running subprocesses can now be terminated gracefully using asyncio's TaskGroup.

Key Changes:

• Migrated from ThreadPoolExecutor to asyncio.TaskGroup for concurrent task execution
• Converted all subprocess calls to use async wrappers for better cancellation handling
• Bumped minimum Python version requirement from 3.10 to 3.11 (required for TaskGroup)

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
utils.py	Added async run() wrapper for subprocess execution with cancellation support and syncify() decorator to bridge async functions with synchronous callers
pyproject.toml	Updated minimum Python version to 3.11 to support asyncio.TaskGroup
pdfservices.py	Converted export() function to async and added yield points for task switching
main.py	Refactored pipeline to use TaskGroup instead of ThreadPoolExecutor, converted all helper functions to async
CHANGELOG.md	Documented the asyncio refactoring and Python version bump

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