* refactor(scanner): remove the legacy ffmpeg metadata extractor
The ffmpeg extractor in scanner/metadata_old has not been wired into the scanner since the taglib-only rewrite, so it was only exercised by its own tests. Remove the package, the FFmpeg.Probe method and its ffmetadata command that only it used, and the startup fallback for Scanner.Extractor="ffmpeg". Configs that still set it keep working: unknown extractors already fall back to taglib with a warning.
* fix(conf): warn and fall back to taglib for an unknown Scanner.Extractor
Validate the option when loading the config, so invalid values such as the removed "ffmpeg" extractor are reported once at startup instead of only when a library storage is created.
bb7d81a5ea