🪲 BUG-#128: Fix Background mode blocking by removing immediate thread.join()

[FernandoCelmer]

Description

• dotflow/core/workflow.py (Background.run): Remove blocking thread.join(), use daemon thread, store thread reference as self.thread
• dotflow/core/workflow.py (Background.get_tasks): Return original self.tasks instead of empty self.queue to avoid race condition
• dotflow/core/workflow.py (Manager.init): Skip _callback_workflow for background mode since tasks have not completed yet
• dotflow/core/workflow.py (Manager.background): Expose process.thread on Manager instance for optional explicit join

Motivation and Context

Background mode called thread.join() immediately after thread.start(), making it identical to sequential mode. Additionally, get_tasks() returned the empty queue (populated asynchronously by the thread), and _callback_workflow fired before any task had run. The fix ensures background mode is truly non-blocking while maintaining access to task objects and thread control.

Closes #128

Types of changes

• Bug fix (change that fixes an issue)
• New feature (change which adds functionality)
• Documentation

Checklist

• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• I have updated the CHANGELOG
• I have updated the documentation accordingly