Block duplicate-release uploads by (title, artist): pre-flight check + server 409 backstop, with within-batch Cut attach via releaseId

DeepDrftManager/Components/Pages/Tracks/BatchEdit.razor

@@ -146,6 +146,9 @@
     private string _releaseDate = string.Empty;
     private ReleaseType _releaseType = ReleaseType.Single;
     private ReleaseMedium _medium = ReleaseMedium.Cut;
+    // The id of the release being edited. New tracks added in this session attach to it via the upload
+    // service's releaseId (ATTACH) path, so they are not rejected as a pre-existing-(title,artist) duplicate.
+    private long? _releaseId;
 
     // The medium selector drives ReleaseType visibility and is persisted on save: every UpdateAsync /
     // UploadTrackAsync call below passes _medium, and PUT api/track/meta resets ReleaseType to its
@@ -214,6 +217,10 @@
         }
 
         var release = tracks[0].Release;
+        // The release being edited already exists, so any new track added here ATTACHES to it (the upload
+        // service's releaseId path) rather than taking the CREATE path, which would reject it as a
+        // duplicate (title, artist). Fall back to the track's own ReleaseId if the nav is not populated.
+        _releaseId = release?.Id ?? tracks[0].ReleaseId;
         _albumName = albumName;
         _artist = release?.Artist ?? string.Empty;
         _genre = release?.Genre ?? string.Empty;
@@ -592,6 +599,7 @@
                             _releaseType,
                             trackNumber,
                             _medium,
+                            _releaseId,
                             progress);
 
                         if (!uploadResult.Success || uploadResult.Value is null)