fix: rewrite concatenate_clips with pure FFmpeg + set IMAGEIO_FFMPEG_EXE in Dockerfile

fix: rewrite apply_copyright_filters with pure FFmpeg - eliminate moviepy silent failures

fix: handle write_videofile failure in apply_copyright_filters + replace os.rename with shutil.copy2

fix: wrong kwarg in create_mixed_audio + cross-platform font paths for Linux Docker

fix: use fresh asyncio event loop to avoid gunicorn thread conflicts

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