fix(audio): support configurable ffmpeg executable

[mohammadaaftabv]

Summary

• Add ffmpeg_executable support for PATH-resolved commands and explicit executable paths.
• Resolve FFmpeg lazily in each executor worker while preserving runtime environments, Ray task scheduling, and positional constructor compatibility.
• Return actionable non-root installation guidance when FFmpeg is unavailable.

Validation

Focused resampling and tutorial/Hydra construction tests pass. Ruff, pre-commit, and DCO checks pass.

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