fix: verify_decode accepts a value that silently skips the decode check

Found by the audit of 2026-09-14.

The decode check after filing runs on the sampled files, on every file, or not at all. An unrecognised verify_decode was stored verbatim and then matched none of those modes, so a typo quietly turned the only end-to-end check of the audio that gets written into a no-op. It is now restricted to all, sample or none.

quality is range-checked at the same time: libvorbis accepts 1-10, and anything outside that reached ffmpeg and failed the whole album with an encoder error instead of a configuration one.