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-core/src/core.rs

@@ -3863,6 +3863,7 @@ mod tests {
         .collect();
         Album {
             favorited_at: None,
+            purchasable: None,
             id: String::new(),
             title: String::new(),
             artist: Artist {
@@ -3900,6 +3901,7 @@ mod tests {
     fn track_with(performer_id: Option<u64>, composer_id: Option<u64>) -> Track {
         Track {
             favorited_at: None,
+            purchasable: None,
             id: 0,
             title: String::new(),
             version: None,

crates/qbz-models/src/types.rs

@@ -518,6 +518,10 @@ pub struct Track {
     /// Favourites-list rows only (unix seconds); see `Album::favorited_at`.
     #[serde(default, deserialize_with = "crate::purchase_serde::lenient_option")]
     pub favorited_at: Option<i64>,
+    /// Store flag from the catalog (`purchasable`): `Some(false)` is the API
+    /// saying this release is not sold on Qobuz; absence is unknown, not no.
+    #[serde(default)]
+    pub purchasable: Option<bool>,
     #[serde(default)]
     pub title: String,
     /// Subtitle/edition info from Qobuz (e.g. "Player's Ball Mix",
@@ -730,6 +734,10 @@ pub struct Album {
     /// it (unix seconds); every other album shape leaves it `None`.
     #[serde(default, deserialize_with = "crate::purchase_serde::lenient_option")]
     pub favorited_at: Option<i64>,
+    /// Store flag from the catalog (`purchasable`): `Some(false)` is the API
+    /// saying this release is not sold on Qobuz; absence is unknown, not no.
+    #[serde(default)]
+    pub purchasable: Option<bool>,
     #[serde(default)]
     pub title: String,
     #[serde(default)]

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,