feat(models): QueueTrack carries isrc/recording_mbid; Track models audio_info

QueueTrack gains two additive, serde-default identity keys (the ISRC and
the MusicBrainz recording id) that the listen log stamps; every
constructor in the workspace sets them (Qobuz-backed ones from
Track.isrc, local ones from the scanned tags, the rest None). They are
JOIN keys, not the row identity: numeric ids collide across sources, so
(source, native item id) stays the identity.

Track.audio_info (replaygain_track_gain / peak on DiscoverAudioInfo) is
modelled from the captured search envelopes; no reader yet and no
popularity field (0 in every sample).

cc

crates/qbz-qt/src/queue_qt.rs

@@ -1538,6 +1538,8 @@ mod tests {
             source_item_id_hint: None,
             context_kind: None,
             context_id: None,
+            isrc: None,
+            recording_mbid: None,
         }
     }