feat(37): reset one scene to its factory snapshot

The per-scene reset restores that scene's settings, layers, webcam placement,
and background-path settings while leaving other scenes' current edits alone.
A focused integration test covers the restore/isolation behavior.
611 tests green, clean build with 0 warnings.

Reference: OBS scene collections store scenes and sources as a saved set:
https://obsproject.com/kb/scene-collections

Services/LayoutStore.Snapshots.cs

@@ -78,6 +78,120 @@ public partial class LayoutStore : IDisposable
             CaptureDefaults();
     }
 
+    /// <summary>Restores one existing scene by its current ID. Unknown scenes have no branded default
+    /// and are left untouched.</summary>
+    public bool RestoreDefaultScene(string sceneId)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(sceneId);
+
+        string? defaultSceneId;
+        using (var find = _connection.CreateCommand())
+        {
+            find.CommandText = """
+                SELECT Id FROM DefaultScene
+                WHERE Name = (SELECT Name FROM Scene WHERE Id = $sceneId) COLLATE NOCASE
+                LIMIT 1;
+                """;
+            find.Parameters.AddWithValue("$sceneId", sceneId);
+            defaultSceneId = find.ExecuteScalar() as string;
+        }
+        if (defaultSceneId == null) return false;
+
+        var columnsByTable = SnapshotTables.ToDictionary(table => table, ReadColumnNames);
+        var webcamId = GetCurrentWebcamId();
+        using var tx = _connection.BeginTransaction();
+
+        using (var clearBackgroundSettings = _connection.CreateCommand())
+        {
+            clearBackgroundSettings.Transaction = tx;
+            clearBackgroundSettings.CommandText = """
+                DELETE FROM Settings
+                WHERE Key IN (
+                    SELECT 'BackgroundPath_' || Id FROM Source WHERE SceneId = $sceneId
+                    UNION SELECT 'BackgroundUseDefault_' || Id FROM Source WHERE SceneId = $sceneId
+                    UNION SELECT 'BackgroundPath_' || $sceneId
+                    UNION SELECT 'BackgroundUseDefault_' || $sceneId
+                );
+                """;
+            clearBackgroundSettings.Parameters.AddWithValue("$sceneId", sceneId);
+            clearBackgroundSettings.ExecuteNonQuery();
+        }
+
+        foreach (var table in new[] { "WebcamSceneConfig", "Source" })
+        {
+            using var delete = _connection.CreateCommand();
+            delete.Transaction = tx;
+            delete.CommandText = $"DELETE FROM {table} WHERE SceneId = $sceneId;";
+            delete.Parameters.AddWithValue("$sceneId", sceneId);
+            delete.ExecuteNonQuery();
+        }
+
+        var sceneColumns = columnsByTable["Scene"]
+            .Where(column => column != "Id" && column != "SortOrder").ToArray();
+        var assignments = string.Join(", ", sceneColumns.Select(column =>
+            $"{QuoteIdentifier(column)} = (SELECT {QuoteIdentifier(column)} FROM DefaultScene WHERE Id = $defaultSceneId)"));
+        using (var updateScene = _connection.CreateCommand())
+        {
+            updateScene.Transaction = tx;
+            updateScene.CommandText = $"UPDATE Scene SET {assignments} WHERE Id = $sceneId;";
+            updateScene.Parameters.AddWithValue("$sceneId", sceneId);
+            updateScene.Parameters.AddWithValue("$defaultSceneId", defaultSceneId);
+            updateScene.ExecuteNonQuery();
+        }
+
+        CopyDefaultSceneRows("Source", sceneId, defaultSceneId, null, tx, columnsByTable["Source"]);
+        if (webcamId != null)
+            CopyDefaultSceneRows("WebcamSceneConfig", sceneId, defaultSceneId, webcamId, tx,
+                columnsByTable["WebcamSceneConfig"]);
+
+        using (var copyBackgroundSettings = _connection.CreateCommand())
+        {
+            copyBackgroundSettings.Transaction = tx;
+            copyBackgroundSettings.CommandText = """
+                INSERT INTO Settings (Key, Value)
+                SELECT Key, Value FROM DefaultLayoutSetting
+                WHERE Key IN (
+                    SELECT 'BackgroundPath_' || Id FROM DefaultSource WHERE SceneId = $defaultSceneId
+                    UNION SELECT 'BackgroundUseDefault_' || Id FROM DefaultSource WHERE SceneId = $defaultSceneId
+                    UNION SELECT 'BackgroundPath_' || $defaultSceneId
+                    UNION SELECT 'BackgroundUseDefault_' || $defaultSceneId
+                );
+                """;
+            copyBackgroundSettings.Parameters.AddWithValue("$defaultSceneId", defaultSceneId);
+            copyBackgroundSettings.ExecuteNonQuery();
+        }
+
+        tx.Commit();
+        return true;
+    }
+
+    private string? GetCurrentWebcamId()
+    {
+        using var cmd = _connection.CreateCommand();
+        cmd.CommandText = "SELECT Id FROM Webcam LIMIT 1;";
+        return cmd.ExecuteScalar() as string;
+    }
+
+    private void CopyDefaultSceneRows(string table, string sceneId, string defaultSceneId,
+        string? webcamId, SqliteTransaction tx, IReadOnlyList<string> columns)
+    {
+        var columnList = string.Join(", ", columns.Select(QuoteIdentifier));
+        var values = string.Join(", ", columns.Select(column => column switch
+        {
+            "SceneId" => "$sceneId",
+            "WebcamId" when webcamId != null => "$webcamId",
+            _ => QuoteIdentifier(column),
+        }));
+
+        using var copy = _connection.CreateCommand();
+        copy.Transaction = tx;
+        copy.CommandText = $"INSERT INTO {table} ({columnList}) SELECT {values} FROM Default{table} WHERE SceneId = $defaultSceneId;";
+        copy.Parameters.AddWithValue("$sceneId", sceneId);
+        copy.Parameters.AddWithValue("$defaultSceneId", defaultSceneId);
+        if (webcamId != null) copy.Parameters.AddWithValue("$webcamId", webcamId);
+        copy.ExecuteNonQuery();
+    }
+
     private void CaptureDefaults()
     {
         var columnsByTable = SnapshotTables.ToDictionary(table => table, ReadColumnNames);