faster-whisper runs on CTranslate2, not torch, but _get_whisper()
imported torch (only to check cuda availability) inside the same try as
the faster-whisper import. on a torch-less machine that raised
ImportError and reported the misleading 'faster-whisper not installed'
even when it was installed, so local mic transcription silently failed.
probe torch separately and optionally: present -> cuda, absent -> cpu.
also declare faster-whisper in requirements-optional.txt (torch stays an
optional extra for gpu).
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