refactor: optimize codebase for better performance and reliability

[namth3]

Key Optimizations:

1. video.py (app/services/video.py)

• Remove unused variable calculation (req_dur)
• Fix resource access after close by saving clip.duration before closing
• Improve error handling in delete_files() with proper logging
• Better code clarity and maintainability

2. material.py (app/services/material.py)

• Fix thread-safety issue: replace global counter with thread-safe implementation using threading.Lock
• Fix resource leak: ensure VideoFileClip is properly closed using try-finally block
• Improve exception handling and resource cleanup
• Prevents potential race conditions in concurrent scenarios

3. video.py controller (app/controllers/v1/video.py)

• Fix critical bug: use correct variable 'file' instead of 'v' in file_to_uri()
• Prevents NameError and ensures proper URI generation for task files

4. utils.py (app/utils/utils.py)

• Improve error logging in to_json() with proper error messages
• Add exc_info=True to background thread error logging for better debugging
• Make background threads non-daemon for more predictable behavior

Impact:

• Better thread-safety for concurrent video generation tasks
• Reduced resource leaks and memory issues
• Improved error visibility and debugging capability
• More robust error handling throughout the codebase
• Fixed potential runtime errors in URI generation

Testing:

• All changes are backward compatible
• No breaking changes to public APIs
• Improved reliability for production use