Three tests in scanner_audio_guard_test.go pin the routes the wipes
traveled; each drives a real scan/rescan/edit over ffmpeg-rendered
fixtures and asserts the payload survives with duration and manifest
intact:
- TestRescanMediaItemPreservesAudioMetadata: folder book through the
focused payload writer.
- TestRescanMediaItemPreservesAudioMetadataSingleFileM4B: the incident
class verbatim — single-file m4b (no layout key) routes through the
generic updater; fixture rendered with ffmpeg AAC/ipod
(writeTestTrackM4B).
- TestMetadataEditorSavePreservesAudioMetadata: the metadata editor's
full-row save through PUT /api/media-items/:id.
Verified to bite: with the handler fix stashed, the editor test fails
with "metadata edit must never NULL audio_metadata"; with it, green.