fix(media): metadata editor save no longer wipes audio_metadata

PUT /api/media-items/:id (the metadata editor) writes the FULL row via
UpdateMediaItem, whose query sets audio_metadata unconditionally — and
audio_metadata has no editor field, so the params literal left it nil
and every save NULLed the payload. A plain metadata edit (genre tweak,
title rename) dropped any audiobook back to "1 second long" with no
track manifest: the same incident class as the scanner wipe, sitting
on the most-used admin path in the app.

Carry existing.AudioMetadata through the write. Write-after-success:
only the audio chain (probe-backed extraction /
UpdateMediaItemAudioBookPayload) ever replaces that column.

internal/handlers/media.go

@@ -1610,6 +1610,14 @@ func (mh *MediaHandler) UpdateMediaItem(c *echo.Context) error {
 		Summary:            pgtype.Text{String: req.Summary, Valid: req.Summary != ""},
 	}
 
+	// audio_metadata is scan-owned (track manifest, chapters, duration)
+	// and has no editor field: the full-row update writes it
+	// unconditionally, so an unguarded save NULLed every audiobook's
+	// payload — books dropped back to "1 second long" on a plain
+	// metadata edit (2026-10-21 incident class). Write-after-success:
+	// carry the existing bytes; only the audio chain replaces them.
+	params.AudioMetadata = existing.AudioMetadata
+
 	// Fields the user actually changed become overrides so future scans keep
 	// the custom values. An explicit cover upload/removal always overrides.
 	overrides := utils.DetectMetadataOverrides(params, existing)