feat(alerts): add scene-local recent ticker

Model the AlertBox and standalone ticker with shared settings; scene tickers render the in-memory active and queued events without saving history. Keep AlertBox content active-event-only.

Ref: https://obsproject.com/kb/obs-studio-overview

Models/Ticker.cs

@@ -0,0 +1,31 @@
+using System.ComponentModel;
+using System.Runtime.CompilerServices;
+
+namespace ytLive.Models;
+
+public sealed class Ticker : INotifyPropertyChanged
+{
+    private bool _isEnabled = true;
+    private AlertDisplayMethod _displayMethod = AlertDisplayMethod.TickerScroll;
+
+    public bool IsEnabled
+    {
+        get => _isEnabled;
+        set => Set(ref _isEnabled, value);
+    }
+
+    public AlertDisplayMethod DisplayMethod
+    {
+        get => _displayMethod;
+        set => Set(ref _displayMethod, value);
+    }
+
+    public event PropertyChangedEventHandler? PropertyChanged;
+
+    private void Set<T>(ref T field, T value, [CallerMemberName] string? propertyName = null)
+    {
+        if (EqualityComparer<T>.Default.Equals(field, value)) return;
+        field = value;
+        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+    }
+}