fix(library): apply per-track artwork fix to second scan loop, suppress legitimate users

A code review of the previous commit surfaced a second scan loop in
legacy_compat.rs that had the same destructive update_album_group_artwork
pattern. Apply the same fix: per-folder cache + drop the destructive
call. Same intent and shape as the main loop fix.

The deprecated marker also surfaced three callers that are NOT bugs
and stay as-is, with #[allow(deprecated)] + a comment to make the
intent explicit:
- CUE ingestion: a CUE sheet is one physical album, so a single cover
  for all CUE tracks is semantically correct.
- v2_library_set_album_artwork: user-initiated set-cover-for-album, by
  design propagates to all tracks.
- Discogs backfill: only fires for albums with no existing artwork, so
  there is nothing per-track to destroy.

cc

src-tauri/src/commands_v2/legacy_compat.rs

@@ -1147,6 +1147,9 @@ async fn library_process_cue_file(
     }
 
     if let (Some(path), false) = (artwork_path.as_ref(), group_key.is_empty()) {
+        // CUE = single album; propagating one cover to all CUE tracks is
+        // semantically correct (they all share the same physical disc).
+        #[allow(deprecated)]
         let _ = db.update_album_group_artwork(&group_key, path);
     }
 
@@ -1602,6 +1605,11 @@ pub async fn v2_library_scan_folder(
         })
         .collect();
 
+    // Per-folder cache for find_folder_artwork (same pattern as the main
+    // scan loop above; replaces the destructive update_album_group_artwork
+    // shortcut that destroyed unique embedded covers).
+    let mut folder_artwork_cache: HashMap<PathBuf, Option<String>> = HashMap::new();
+
     for audio_path in &scan_result.audio_files {
         if state.scan_cancel.load(Ordering::Relaxed) {
             let mut progress = state.scan_progress.lock().await;
@@ -1631,32 +1639,37 @@ pub async fn v2_library_scan_folder(
                 let mut artwork_path =
                     MetadataExtractor::extract_artwork(&canonical_path, &artwork_cache);
                 if artwork_path.is_none() {
-                    if let Some(folder_art) = MetadataExtractor::find_folder_artwork(
-                        canonical_path.as_path(),
-                        Some(&track.album),
-                    ) {
+                    let folder = canonical_path
+                        .parent()
+                        .map(|p| p.to_path_buf())
+                        .unwrap_or_else(|| canonical_path.clone());
+                    let cached = folder_artwork_cache
+                        .entry(folder)
+                        .or_insert_with(|| {
+                            MetadataExtractor::find_folder_artwork(
+                                canonical_path.as_path(),
+                                Some(&track.album),
+                            )
+                        })
+                        .clone();
+                    if let Some(folder_art) = cached {
                         artwork_path = MetadataExtractor::cache_artwork_file(
                             std::path::Path::new(&folder_art),
                             &artwork_cache,
                         );
                     }
                 }
-                track.artwork_path = artwork_path.clone();
+                track.artwork_path = artwork_path;
 
                 let guard__ = state.db.lock().await;
                 let db = guard__
                     .as_ref()
                     .ok_or("No active session - please log in")?;
-                let group_key = track.album_group_key.clone();
                 if let Err(e) = db.insert_track(&track) {
                     all_errors.push(ScanError {
                         file_path: path_str,
                         error: e.to_string(),
                     });
-                } else if let Some(path) = artwork_path.as_ref() {
-                    if !group_key.is_empty() {
-                        let _ = db.update_album_group_artwork(&group_key, path);
-                    }
                 }
             }
             Err(e) => {

src-tauri/src/commands_v2/library.rs

@@ -1148,6 +1148,9 @@ pub async fn v2_library_set_album_artwork(
         .ok_or_else(|| "Failed to cache artwork file".to_string())?;
     let guard = state.db.lock().await;
     let db = guard.as_ref().ok_or("No active session - please log in")?;
+    // User-initiated set-album-artwork command: writing one cover to all
+    // tracks in the group is the explicit user intent here.
+    #[allow(deprecated)]
     db.update_album_group_artwork(&album_group_key, &cached_path)
         .map_err(|e| e.to_string())?;
     Ok(cached_path)
@@ -2742,6 +2745,9 @@ pub async fn v2_library_fetch_missing_artwork(
             let db = guard__
                 .as_ref()
                 .ok_or("No active session - please log in")?;
+            // Backfill: only runs for albums with NO existing artwork, so there's
+            // nothing per-track to destroy. Group-level write is correct here.
+            #[allow(deprecated)]
             if db
                 .update_album_group_artwork(&group_key, &artwork_path)
                 .is_ok()