perf: separate library DB connection for download post-processing

Add a second LibraryDatabase connection to OfflineCacheState dedicated
to download post-processing writes. SQLite WAL mode natively handles
concurrent reads (UI via main connection) and writes (downloads via
this separate connection), eliminating app-level mutex contention that
caused progressive UI lag during bulk downloads.

Init after library.init_at() in both activate_session and
activate_offline_session. Teardown before main offline_cache teardown.

src-tauri/src/commands_v2.rs

@@ -6549,7 +6549,6 @@ pub async fn v2_cache_track_for_offline(
     sample_rate: Option<f64>,
     state: State<'_, AppState>,
     cache_state: State<'_, OfflineCacheState>,
-    library_state: State<'_, crate::library::LibraryState>,
     app_handle: tauri::AppHandle,
 ) -> Result<(), String> {
     log::info!("Command: v2_cache_track_for_offline {} - {} by {}", track_id, title, artist);
@@ -6581,7 +6580,7 @@ pub async fn v2_cache_track_for_offline(
         cache_state.fetcher.clone(),
         cache_state.db.clone(),
         cache_state.get_cache_path(),
-        library_state.db.clone(),
+        cache_state.library_db.clone(),
         app_handle.clone(),
         cache_state.cache_semaphore.clone(),
     );
@@ -6608,7 +6607,6 @@ pub async fn v2_cache_tracks_batch_for_offline(
     tracks: Vec<BatchTrackInfo>,
     state: State<'_, AppState>,
     cache_state: State<'_, OfflineCacheState>,
-    library_state: State<'_, crate::library::LibraryState>,
     app_handle: tauri::AppHandle,
 ) -> Result<(), String> {
     log::info!("Command: v2_cache_tracks_batch_for_offline ({} tracks)", tracks.len());
@@ -6652,7 +6650,7 @@ pub async fn v2_cache_tracks_batch_for_offline(
             cache_state.fetcher.clone(),
             cache_state.db.clone(),
             cache_state.get_cache_path(),
-            library_state.db.clone(),
+            cache_state.library_db.clone(),
             app_handle.clone(),
             cache_state.cache_semaphore.clone(),
         );

src-tauri/src/offline_cache/mod.rs

@@ -135,6 +135,9 @@ pub struct OfflineCacheState {
     /// Cache limit in bytes (None = unlimited)
     pub limit_bytes: Arc<Mutex<Option<u64>>>,
     pub cache_semaphore: Arc<Semaphore>,
+    /// Separate library DB connection for download post-processing writes.
+    /// This avoids contending with the main library DB mutex used by UI queries.
+    pub library_db: Arc<Mutex<Option<qbz_library::LibraryDatabase>>>,
 }
 
 impl OfflineCacheState {
@@ -165,6 +168,7 @@ impl OfflineCacheState {
             cache_dir: Arc::new(RwLock::new(cache_dir.clone())),
             limit_bytes: Arc::new(Mutex::new(default_limit)),
             cache_semaphore: Arc::new(Semaphore::new(3)),
+            library_db: Arc::new(Mutex::new(None)),
         };
 
         log::info!("Offline cache initialized at: {:?}", cache_dir);
@@ -183,6 +187,7 @@ impl OfflineCacheState {
             cache_dir: Arc::new(RwLock::new(cache_dir)),
             limit_bytes: Arc::new(Mutex::new(Some(2 * 1024 * 1024 * 1024u64))),
             cache_semaphore: Arc::new(Semaphore::new(3)),
+            library_db: Arc::new(Mutex::new(None)),
         }
     }
 
@@ -206,7 +211,24 @@ impl OfflineCacheState {
         Ok(())
     }
 
+    /// Open a separate library DB connection for download post-processing.
+    /// Must be called after library.init_at() so the schema exists.
+    pub async fn init_library_connection(&self, data_dir: &std::path::Path) -> Result<(), String> {
+        let db_path = data_dir.join("library.db");
+        let lib_db = qbz_library::LibraryDatabase::open(&db_path)
+            .map_err(|e| format!("Failed to open download library connection: {}", e))?;
+        let mut guard = self.library_db.lock().await;
+        *guard = Some(lib_db);
+        log::info!("Offline cache: separate library DB connection opened at {:?}", db_path);
+        Ok(())
+    }
+
     pub async fn teardown(&self) {
+        // Close library connection first (before main teardown)
+        {
+            let mut lib_guard = self.library_db.lock().await;
+            *lib_guard = None;
+        }
         let mut guard = self.db.lock().await;
         *guard = None;
     }

src-tauri/src/session_lifecycle.rs

@@ -134,6 +134,7 @@ pub async fn activate_session(app: &tauri::AppHandle, user_id: u64) -> Result<()
 
     // Cache-dir stores
     offline_cache.init_at(&cache_dir).await?;
+    offline_cache.init_library_connection(&data_dir).await?;
     lyrics.init_at(&cache_dir).await?;
 
     // Run deferred subscription purge check
@@ -313,6 +314,7 @@ pub async fn activate_offline_session(app: &tauri::AppHandle) -> Result<(), Stri
     library.init_at(&data_dir).await?;
     offline.init_at(&data_dir)?;
     offline_cache.init_at(&cache_dir).await?;
+    offline_cache.init_library_connection(&data_dir).await?;
     audio_settings.init_at(&data_dir)?;
     playback_prefs.init_at(&data_dir)?;