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.Migrations.cs

@@ -285,6 +285,40 @@ public partial class LayoutStore : IDisposable
             cmd.ExecuteNonQuery();
         }
 
+        // TASK 47: alert video config. The default clip asset id + the creator's
+        // replacement path (a FILE reference, never a stored BLOB) + the pill,
+        // ticker toggle and volume live on the row.
+        if (!columns.Contains("AlertVideoAssetId"))
+        {
+            using var cmd = _connection.CreateCommand();
+            cmd.CommandText = "ALTER TABLE Source ADD COLUMN AlertVideoAssetId TEXT;";
+            cmd.ExecuteNonQuery();
+        }
+        if (!columns.Contains("AlertVideoPath"))
+        {
+            using var cmd = _connection.CreateCommand();
+            cmd.CommandText = "ALTER TABLE Source ADD COLUMN AlertVideoPath TEXT;";
+            cmd.ExecuteNonQuery();
+        }
+        if (!columns.Contains("AlertUseDefaultVideo"))
+        {
+            using var cmd = _connection.CreateCommand();
+            cmd.CommandText = "ALTER TABLE Source ADD COLUMN AlertUseDefaultVideo INTEGER NOT NULL DEFAULT 1;";
+            cmd.ExecuteNonQuery();
+        }
+        if (!columns.Contains("AlertShowTicker"))
+        {
+            using var cmd = _connection.CreateCommand();
+            cmd.CommandText = "ALTER TABLE Source ADD COLUMN AlertShowTicker INTEGER NOT NULL DEFAULT 1;";
+            cmd.ExecuteNonQuery();
+        }
+        if (!columns.Contains("AlertVideoVolume"))
+        {
+            using var cmd = _connection.CreateCommand();
+            cmd.CommandText = "ALTER TABLE Source ADD COLUMN AlertVideoVolume REAL NOT NULL DEFAULT 1;";
+            cmd.ExecuteNonQuery();
+        }
+
         // Rename legacy "Backdrop" display name to "Background" in existing rows
         using (var cmd = _connection.CreateCommand())
         {