test(integration): route regression tests for the audio_metadata guards

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.

cmd/server/tests/audiobook_multifile_test.go

@@ -47,6 +47,24 @@ func writeTestTrack(t *testing.T, path string) {
 	require.NoError(t, os.WriteFile(path, []byte("ID3 dummy audio"), 0o644))
 }
 
+// writeTestTrackM4B is writeTestTrack for the m4b (AAC/ipod muxer)
+// container — the audiobook format whose rescan route is under guard.
+// ffmpeg needs -f ipod for the .m4b extension; falls back to dummy bytes.
+func writeTestTrackM4B(t *testing.T, path string) {
+	t.Helper()
+	require.NoError(t, os.MkdirAll(filepath.Dir(path), 0o755))
+	if ffmpeg, err := exec.LookPath("ffmpeg"); err == nil {
+		cmd := exec.Command(ffmpeg, "-y", "-v", "error", "-f", "lavfi",
+			"-i", "sine=frequency=440:duration=0.3", "-codec:a", "aac", "-b:a", "32k", "-f", "ipod", path)
+		if out, err := cmd.CombinedOutput(); err == nil {
+			return
+		} else {
+			t.Logf("ffmpeg m4b render failed (%s), falling back to dummy bytes: %s", err, out)
+		}
+	}
+	require.NoError(t, os.WriteFile(path, []byte("dummy m4b audio"), 0o644))
+}
+
 // audioTestLibrary builds an audiobooks-type library with a fresh temp
 // folder, returning its UUID and the folder path.
 func audioTestLibrary(t *testing.T, setup *TestServerSetup, name string) (pgtype.UUID, string) {