fix: clear actionable error when ffmpeg/ffprobe is missing

[seonghobae]

Problem

When ffmpeg/ffprobe is not installed or not on PATH, probe_media and detect_silence_intervals let subprocess.run raise a bare FileNotFoundError:

FileNotFoundError: [Errno 2] No such file or directory: 'ffprobe'

That is a raw traceback with zero guidance. Because probe_media runs first in the conversion pipeline, this is the error users actually hit when ffmpeg isn't installed — long before reaching _execute_plan, which was already guarded. "ffmpeg not installed" is one of the most common user failures, so a clear message is genuine value.

Fix

• Add a shared _run_media_tool() helper that runs an ffmpeg/ffprobe command and maps a missing binary to a MediaShrinkerError that names the tool and how to install it (brew install ffmpeg / sudo apt install ffmpeg).
• Route all three ffmpeg/ffprobe invocation sites (probe_media, detect_silence_intervals, _execute_plan) through it. This also unifies and improves _execute_plan's previously terse ffmpeg not found: <path> message.
• Nonzero-exit / stderr handling stays at each call site; only the tool-invocation FileNotFoundError path changes. No parsing, bitrate, or output-validation logic touched.

Tests

TDD: added two tests asserting a clear MediaShrinkerError (naming the binary + "Install ffmpeg") when subprocess.run raises FileNotFoundError for missing ffprobe (probe_media) and missing ffmpeg (detect_silence_intervals). Both fail on the old raw-FileNotFoundError behavior.

Gates

• coverage report --include=media_shrinker.py: 100%
• interrogate media_shrinker.py: 100%
• Full suite: no new failures (only the 5 pre-existing macOS os.listxattr errors remain)

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