Discovery of the external ffmpeg/ffprobe binaries.
Search order (see docs/PRD/03-tech-stack.md):
- Explicit path from an env var (
DEEPSHRINK_FFMPEG/DEEPSHRINK_FFPROBE). - A binary in
PATH. - (later) a bundled binary next to the app — for the desktop layer.
If not found, [FfmpegError::NotFound]; the CLI maps this to exit code 3
with an install hint. ffprobe/progress parsing arrives in sessions 002/003.