feat(alerts): TASK 47 — alert box plays a video (built-in/custom clip) + read-time fade + message ticker

TASK 43's alert box grows a real video celebration. Per-alert IAlertClipDecoder
(ffmpeg bgra + f32le pipes, real-time paced, disposed at drain) plays the shipped
Assets/alert-default.mp4 (stamped into the Asset table at startup) unless the
creator picks their own file — path reference only, never stored in the DB; the
six AlertRenderer animations stay the fallback. ~0.3s fade rides the alpha
envelope on straight-source copies (EOF freeze-frames then fades out); audio
forwards to a new AudioMixer alert ring (8s, 48k stereo) drained at unity — no
duck, creator ruling — scaled by volume × fade. An auto-composed marquee ticker
('Funder — Super Chat · $10.00', 140px/s) scrolls top-of-frame via a
FramePump._alertTicker seam through Render/CompositeLayers, mixed into the cache
signature (dynamic overlay, never baked). New Stream Alerts section in LeftPanel.

Derivative-work references (how OBS/Streamlabs alert boxes do per-alert video):
- https://support.streamlabs.com/hc/en-us/articles/217741147-Setting-Up-Your-Streamlabs-Alerts (custom image/video per alert type + variations)
- https://obsproject.com/kb/stream-tutorial-2-alerts (alert overlay as an on-screen zone)
- https://streamlabs.com/content-hub/widgets/alert-box (per-event alert playback)

Good Dog: AlertLayerVideoTests drives a fake IAlertClipDecoder through the whole
lifecycle in one pass (custom path wins, decoder spawns/disposes, fade envelope
0→127→255, audio volume×fade, ticker scrolls, EOF fade-drain to idle). It caught
the clip branch of Advance not clearing _current before AdvanceToNext — the layer
stayed IsPlaying after drain (MyMistakes post-mortem).

Full vstest 319/319; clean build 0 warnings; scope check green.

Models/Source.cs

@@ -179,6 +179,47 @@ public class Source : SceneElement
 
     public bool IsMediaSource => Type == SourceType.MediaSource;
 
+    // Alert video config (TASK 47): the shipped default clip lives in the Asset
+    // table (seeded like image assets); the creator's replacement is a FILE PATH
+    // reference only — never into the DB (creator ruling, 2026-09-26). Runtime
+    // order: replacement path exists → DB default → six celebration animations.
+    private string? _alertVideoAssetId;
+    public string? AlertVideoAssetId
+    {
+        get => _alertVideoAssetId;
+        set => Set(ref _alertVideoAssetId, value);
+    }
+
+    private string? _alertVideoPath;
+    public string? AlertVideoPath
+    {
+        get => _alertVideoPath;
+        set => Set(ref _alertVideoPath, value);
+    }
+
+    private bool _alertUseDefaultVideo = true;
+    public bool AlertUseDefaultVideo
+    {
+        get => _alertUseDefaultVideo;
+        set => Set(ref _alertUseDefaultVideo, value);
+    }
+
+    private bool _alertShowTicker = true;
+    public bool AlertShowTicker
+    {
+        get => _alertShowTicker;
+        set => Set(ref _alertShowTicker, value);
+    }
+
+    private double _alertVideoVolume = 1.0;
+    public double AlertVideoVolume
+    {
+        get => _alertVideoVolume;
+        set => Set(ref _alertVideoVolume, Math.Clamp(value, 0.0, 1.0));
+    }
+
+    public bool IsAlertBox => Type == SourceType.AlertBox;
+
     // Chat box config (TASK 3.18)
     private double _chatFontSize = 16;
     public double ChatFontSize