Flag slave-only songs whose album or artist the master has

Scans now mark slave-only songs as missing from the master when the master
has their album folder or at least their artist folder. A new Missing from
master filter in Review lists them, split by album and artist.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/export.go

@@ -63,6 +63,10 @@ func recommendation(m store.Match, v probe.Verdict) string {
 		return "review"
 	case m.Status == store.StatusError:
 		return "fix error"
+	case m.Missing == "album":
+		return "not in master: master has the album"
+	case m.Missing == "artist":
+		return "not in master: master has the artist"
 	}
 	return ""
 }