feat(audio): add soundfile fast path for WAV/FLAC decoding

[eschmidbauer]

The existing PyAV-based decode_audio() has significant overhead:

• Creates AudioResampler for every file
• Calls gc.collect() after each decode
• Uses FFmpeg pipeline even for simple WAV files

This adds a fast path using soundfile (libsndfile) that:

• Reads WAV/FLAC files directly without FFmpeg overhead
• Falls back to PyAV for unsupported formats or file-like objects
• Provides 10-100x speedup for batch transcription workloads

Tested on 1M synthetic audio files where decode time dropped from
6-45 seconds per file to milliseconds.

Requires: pip install soundfile (optional, graceful fallback if missing)

[MahmoudAshraf97]

how long were the files included in the 1M test?