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.Load.cs

@@ -99,7 +99,8 @@ public partial class LayoutStore : IDisposable
                        IsBackground, CaptureKey, ShowDesktop,
                        ChatFontSize, ChatFontColor, ChatBackgroundColor, ChatMaxMessages,
                        ChatShowTimestamps, ChatShowBadges, ChatMemberColor, ChatSuperChatColor,
-                       WebUri, MediaPath, MediaIsLooping, MediaVolume, MediaPlaybackState, SortOrder
+                       WebUri, MediaPath, MediaIsLooping, MediaVolume, MediaPlaybackState,
+                       AlertVideoAssetId, AlertVideoPath, AlertUseDefaultVideo, AlertShowTicker, AlertVideoVolume, SortOrder
                 FROM Source ORDER BY SortOrder
                 """;
             using var reader = cmd.ExecuteReader();
@@ -145,10 +146,15 @@ public partial class LayoutStore : IDisposable
                         : Enum.TryParse<MediaPlaybackState>(reader.GetString(29), out var ps)
                             ? ps
                             : MediaPlaybackState.Stopped,
+                    AlertVideoAssetId = reader.IsDBNull(30) ? null : reader.GetString(30),
+                    AlertVideoPath = reader.IsDBNull(31) ? null : reader.GetString(31),
+                    AlertUseDefaultVideo = reader.IsDBNull(32) ? true : reader.GetInt32(32) != 0,
+                    AlertShowTicker = reader.IsDBNull(33) ? true : reader.GetInt32(33) != 0,
+                    AlertVideoVolume = reader.IsDBNull(34) ? 1.0 : reader.GetDouble(34),
                 };
                 if (!sourcesByScene.TryGetValue(sceneId, out var list))
                     sourcesByScene[sceneId] = list = new List<(Source, int)>();
-                list.Add((source, reader.GetInt32(30)));
+                list.Add((source, reader.GetInt32(35)));
             }
         }