fix: match audio and video extensions case-insensitively

[Dhevenddra]

Problem

list_files() filters by extension with an exact match:

files = [f for f in files if f.suffix in extensions]

Path.suffix preserves case, and AUDIO_EXTENSIONS and VIDEO_EXTENSIONS are all lowercase. A file named RECORDING.WAV therefore never matches, and list_files drops it without a warning.

Recorders, field recorders, phones and camera firmware routinely write uppercase extensions, so this hits real datasets. Every command that scans a directory is affected, because they all pass those lowercase sets: length, loudness-norm, merge-short, resample, separate-audio, slice-audio, convert-to-wav, merge-lab and transcribe.

The failure is silent. The files are simply absent from the count, so a run over a directory of .WAV files reports success having processed nothing.

Reproduced against main with six files in one directory:

files on disk : ['Mixed.Wav', 'UPPER.MP3', 'UPPER.WAV', 'ignored.txt', 'lower.mp3', 'lower.wav']
list_files got: ['lower.mp3', 'lower.wav']

MISSING (silently skipped): ['Mixed.Wav', 'UPPER.MP3', 'UPPER.WAV']

After the change all five audio files are found and ignored.txt is still excluded.

Fix

Compare case-insensitively. list_files also lowercases the incoming set, so a caller passing {".WAV"} behaves the same way.

cli/merge_lab.py and cli/transcribe.py re-filter on AUDIO_EXTENSIONS after calling list_files, so they carried the same comparison and are updated too.

Notes

Only the extension comparison changed. Sorting, recursion, the video/audio sets and the returned Path objects keep their existing behaviour, and the original case of each filename is untouched.

The repository has no test suite or test dependency, so rather than introduce one I verified with the directory reproduction above, run before and after the change. Happy to add tests if you would like a framework brought in.

black --check, isort --check-only and codespell are all clean.