Migrate remaining legacy operator workflows

src/MBN_STOCK_WEBVIEW.Playout/Configuration/PlayoutOptions.cs

@@ -26,7 +26,7 @@ public sealed class PlayoutOptions
 
     /// <summary>
     /// Trusted MainForm-level background selection. The asset is always a path relative
-    /// to SceneDirectory and is never accepted from Web content.
+    /// to LegacyBackgroundDirectory and is never accepted from Web content.
     /// </summary>
     public LegacySceneBackgroundKind LegacySceneBackgroundKind { get; set; } =
         LegacySceneBackgroundKind.None;
@@ -37,6 +37,14 @@ public sealed class PlayoutOptions
 
     public bool LegacySceneBackgroundVideoLoopInfinite { get; set; } = true;
 
+    /// <summary>
+    /// External absolute root containing operator-selectable MainForm backgrounds.
+    /// When omitted, the original layout is retained by deriving a sibling "배경"
+    /// directory from SceneDirectory (Cuts\..\배경). This root is never merged with
+    /// the trusted scene/Cuts root.
+    /// </summary>
+    public string? LegacyBackgroundDirectory { get; set; }
+
     /// <summary>
     /// External absolute root containing vendor .t2s scenes. Required in Test and Live modes.
     /// </summary>