Optimize performance with parallel processing and faster compression settings

[munsonbh]

Summary

This PR implements several performance optimizations to significantly speed up image processing:

Changes

• Parallel image processing: Processes multiple images concurrently using threading (default: CPU core count)
• Parallel format conversion: Converts to JPEG XL and WebP simultaneously for each image
• Optimized compression settings:
  • JPEG XL effort reduced from 9 to 7 (faster with minimal size difference)
  • WebP method reduced from 6 to 4 (faster with minimal size difference)
• Smart skipping: Automatically skips files already in optimized formats (JXL/WebP)
• New flag: Added --workers flag to control parallelism

Performance Improvements

• Individual images: ~2-3x faster per image (lower compression effort + parallel conversions)
• Overall throughput: ~4-8x faster with parallel processing (depends on CPU cores)
• Example: Processing 76 images could go from ~85 minutes to ~10-15 minutes on an 8-core system

Testing

• All existing functionality preserved
• Compression quality maintained (minimal size difference with faster settings)
• Thread-safe implementation with proper error handling

Documentation

• Updated README with performance optimizations section
• Added usage examples for --workers flag

[munsonbh]

Closing this PR to fix an issue with exception logging.

Issue Found:
The exc_info=True parameter in logger.error() was being used outside of an exception handler context (in the result processing loop). The exc_info=True parameter only works within an active exception handler context (inside an except block), so it was ineffective and wouldn't include exception traceback information as intended.

Fix Applied:

• Moved exception logging with exc_info=True to the worker thread where the exception context exists
• Removed exc_info=True from the main thread result processing loop where we only have the error message string
• Exception traceback is now properly logged in the worker thread where exceptions occur

A new PR will be opened with this fix included.