feat(models): albums and tracks carry the catalog's purchasable flag

Qobuz answers `purchasable` on every album and track object (captured in
the API samples); the models never read it. `Some(false)` is the catalog
saying a release is not sold, absence stays unknown. The exhaustive test
fixtures in qbz-core and qbz-playlist-import spell the field out.

cc

crates/qbz-playlist-import/src/match_qobuz.rs

@@ -393,6 +393,7 @@ mod tests {
     fn qobuz_track(id: u64, title: &str, artist: &str) -> Track {
         Track {
             favorited_at: None,
+            purchasable: None,
             id,
             title: title.to_string(),
             version: None,