fix(reco): route Recommendations MusicBrainz lookups through the core client

recommendations_qt and external_reco_qt built private MusicBrainzClient
instances, which start enabled and ignore the musicbrainz_enabled
opt-out that integrations_qt applies to the core's client. Expose the
core's Arc<MusicBrainzClient> and borrow it instead, so a disabled
MusicBrainz sends zero requests from Discover > Recommendations. The tag
editor's three private clients stay: those are explicit manual lookups
the user triggers by hand.

cc

crates/qbz-core/src/core.rs

@@ -2243,6 +2243,14 @@ impl<A: FrontendAdapter + Send + Sync + 'static> QbzCore<A> {
         self.musicbrainz.set_enabled(enabled).await;
     }
 
+    /// The core's shared MusicBrainz client — the ONE instance whose enabled
+    /// flag `musicbrainz_set_enabled` toggles. Frontend code that needs a
+    /// client must borrow this one: a private `MusicBrainzClient::new()`
+    /// starts enabled and silently ignores the user's opt-out.
+    pub fn musicbrainz_client(&self) -> Arc<MusicBrainzClient> {
+        Arc::clone(&self.musicbrainz)
+    }
+
     /// Resolve an artist name to a MusicBrainz id. Returns `None` if no
     /// confident match is found.
     pub async fn musicbrainz_resolve_artist(

crates/qbz-qt/src/external_reco_qt.rs

@@ -23,7 +23,6 @@ use qbz_external_reco::{
     RecoInputs,
 };
 use qbz_integrations::lastfm::LastFmClient;
-use qbz_integrations::musicbrainz::MusicBrainzClient;
 use qbz_models::{Album, Artist, Track};
 
 /// Same 30-day TTL as the Slint side.
@@ -150,7 +149,9 @@ pub async fn similar_albums(
     }
 
     let lastfm_client = LastFmClient::new();
-    let mb_client = MusicBrainzClient::new();
+    // The core's client (respects the MusicBrainz opt-out) — see the same
+    // note in `recommendations_qt::run`.
+    let mb_client = runtime.core().musicbrainz_client();
     let catalog = CoreRecoCatalog {
         runtime: runtime.clone(),
     };
@@ -160,7 +161,7 @@ pub async fn similar_albums(
             client: &lastfm_client,
         }),
         listenbrainz: None,
-        musicbrainz: &mb_client,
+        musicbrainz: mb_client.as_ref(),
         catalog: &catalog,
         cache: cache.as_ref(),
         local: LocalHistory::default(),

crates/qbz-qt/src/recommendations_qt.rs

@@ -62,7 +62,6 @@ use qbz_external_reco::{
 };
 use qbz_integrations::lastfm::LastFmClient;
 use qbz_integrations::listenbrainz::ListenBrainzClient;
-use qbz_integrations::musicbrainz::MusicBrainzClient;
 use qbz_models::Artist;
 
 use crate::home_qt::{HomeCard, HomeSection};
@@ -669,7 +668,11 @@ async fn run(force: bool) {
             )
             .await;
     }
-    let mb_client = MusicBrainzClient::new();
+    // The core's client, not a private one: only the core instance carries
+    // the `musicbrainz_enabled` opt-out (integrations_qt::start applies it),
+    // and a disabled client answers every lookup with an error the builders
+    // already treat as "no MusicBrainz" — zero requests leave the machine.
+    let mb_client = crate::app().core().musicbrainz_client();
 
     // CONNECTED, not merely enabled: a `None` handle is what makes the row
     // absent AND silent (the builders early-return before any request).
@@ -717,7 +720,7 @@ async fn run(force: bool) {
     let inputs = RecoInputs {
         lastfm,
         listenbrainz,
-        musicbrainz: &mb_client,
+        musicbrainz: mb_client.as_ref(),
         catalog: &catalog,
         cache: cache.as_ref(),
         local: LocalHistory {