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

Services/Encoder/FramePump.cs

@@ -29,7 +29,7 @@ public sealed class FramePump : IDisposable
     private readonly Action<string>? _log;
     private readonly Func<TimeSpan, CancellationToken, Task> _pacingDelay;
     private readonly Func<(VideoFrame? Frame, SocialBarPosition Position)>? _socialBar;
-    private readonly Func<VideoFrame?>? _alertTicker;
+    private readonly Func<Scene, VideoFrame?>? _alertTicker;
     private readonly Func<VideoFrame?>? _brandFlash;
     private readonly TransitionService? _transition;
     private VideoFrame? _lastOutput;
@@ -138,7 +138,7 @@ public sealed class FramePump : IDisposable
         Action<string>? log = null,
         Func<TimeSpan, CancellationToken, Task>? pacingDelay = null,
         Func<(VideoFrame? Frame, SocialBarPosition Position)>? socialBar = null,
-        Func<VideoFrame?>? alertTicker = null,
+        Func<Scene, VideoFrame?>? alertTicker = null,
         Func<VideoFrame?>? brandFlash = null,
         TransitionService? transition = null,
         SceneGraph? sceneGraph = null)
@@ -486,7 +486,7 @@ public sealed class FramePump : IDisposable
                     }
 
                     var sw = System.Diagnostics.Stopwatch.StartNew();
-                    VideoFrame? tickerFrame = _alertTicker?.Invoke();
+                    VideoFrame? tickerFrame = _alertTicker?.Invoke(scene);
                     tickerTicks += sw.ElapsedTicks;
                     sw.Restart();
                     VideoFrame? flashFrame = _brandFlash?.Invoke();