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.

Services/LayoutStore.Save.cs

@@ -88,13 +88,15 @@ public partial class LayoutStore : IDisposable
                                     ClipShape, IsMirrored, IsBackground, CaptureKey, ShowDesktop, SortOrder,
                                     ChatFontSize, ChatFontColor, ChatBackgroundColor, ChatMaxMessages,
                                     ChatShowTimestamps, ChatShowBadges, ChatMemberColor, ChatSuperChatColor,
-                                    WebUri, MediaPath, MediaIsLooping, MediaVolume, MediaPlaybackState)
+                                    WebUri, MediaPath, MediaIsLooping, MediaVolume, MediaPlaybackState,
+                                    AlertVideoAssetId, AlertVideoPath, AlertUseDefaultVideo, AlertShowTicker, AlertVideoVolume)
                 VALUES ($id, $sceneId, $assetId, $type, $name, $isEnabled,
                         $x, $y, $w, $h, $opacity, $monitor,
                         $clip, $mirrored, $isBackground, $captureKey, $showDesktop, $sort,
                         $chatFontSize, $chatFontColor, $chatBgColor, $chatMaxMsgs,
                         $chatTimestamps, $chatBadges, $chatMemberColor, $chatSuperChatColor,
-                        $webUri, $mediaPath, $mediaIsLooping, $mediaVolume, $mediaPlaybackState)
+                        $webUri, $mediaPath, $mediaIsLooping, $mediaVolume, $mediaPlaybackState,
+                        $alertVideoAssetId, $alertVideoPath, $alertUseDefaultVideo, $alertShowTicker, $alertVideoVolume)
                 """;
             cmd.Transaction = tx;
             var idP = cmd.Parameters.Add("$id", SqliteType.Text);
@@ -128,6 +130,11 @@ public partial class LayoutStore : IDisposable
             var mediaIsLoopingP = cmd.Parameters.Add("$mediaIsLooping", SqliteType.Integer);
             var mediaVolumeP = cmd.Parameters.Add("$mediaVolume", SqliteType.Real);
             var mediaPlaybackStateP = cmd.Parameters.Add("$mediaPlaybackState", SqliteType.Text);
+            var alertVideoAssetIdP = cmd.Parameters.Add("$alertVideoAssetId", SqliteType.Text);
+            var alertVideoPathP = cmd.Parameters.Add("$alertVideoPath", SqliteType.Text);
+            var alertUseDefaultVideoP = cmd.Parameters.Add("$alertUseDefaultVideo", SqliteType.Integer);
+            var alertShowTickerP = cmd.Parameters.Add("$alertShowTicker", SqliteType.Integer);
+            var alertVideoVolumeP = cmd.Parameters.Add("$alertVideoVolume", SqliteType.Real);
 
             // Source and WebcamSceneConfig share ONE z-order space per scene:
             // each element gets its index within scene.Elements regardless of
@@ -170,6 +177,11 @@ public partial class LayoutStore : IDisposable
                     mediaIsLoopingP.Value = source.MediaIsLooping ? 1 : 0;
                     mediaVolumeP.Value = source.MediaVolume;
                     mediaPlaybackStateP.Value = source.MediaPlaybackState.ToString();
+                    alertVideoAssetIdP.Value = (object?)source.AlertVideoAssetId ?? DBNull.Value;
+                    alertVideoPathP.Value = (object?)source.AlertVideoPath ?? DBNull.Value;
+                    alertUseDefaultVideoP.Value = source.AlertUseDefaultVideo ? 1 : 0;
+                    alertShowTickerP.Value = source.AlertShowTicker ? 1 : 0;
+                    alertVideoVolumeP.Value = source.AlertVideoVolume;
                     cmd.ExecuteNonQuery();
                 }
             }
@@ -300,7 +312,15 @@ public partial class LayoutStore : IDisposable
 
         using (var cmd = _connection.CreateCommand())
         {
-            cmd.CommandText = "DELETE FROM Asset WHERE Id NOT IN (SELECT AssetId FROM Source WHERE AssetId IS NOT NULL);";
+            cmd.CommandText = """
+                DELETE FROM Asset WHERE Id NOT IN (
+                    SELECT AssetId FROM Source WHERE AssetId IS NOT NULL
+                    UNION
+                    SELECT AlertVideoAssetId FROM Source WHERE AlertVideoAssetId IS NOT NULL
+                    UNION
+                    SELECT Value FROM Settings WHERE Key = 'AlertDefaultVideoAssetId'
+                );
+                """;
             cmd.Transaction = tx;
             cmd.ExecuteNonQuery();
         }