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

docs/ai/overlays.md

@@ -98,16 +98,24 @@ published decisions (recorded in `TASKS/task-43-native-alerts.md`):
   EOF → freeze-frame → fade-out → drain (the clip branch of `Advance` **clears
   `_current` before `AdvanceToNext`** — the miss that the TASK 47 test caught). Audio
   forwards to the mixer sink scaled by `volume × fade`.
-- **Alert ticker (announcement strip):** `AlertTickerFrame` (in the layer) composes
+- **Ticker settings and alert feed:** `Models/Ticker.cs` is shared by the
+  AlertBox and standalone `SourceType.Ticker` layers. Each source owns a distinct
+  settings instance (`IsEnabled` + `DisplayMethod`); the existing `AlertShowTicker`
+  and `AlertDisplayMethod` columns persist those settings on their own Source row,
+  so this is additive without a schema migration. The standalone ticker can be
+  added once in each scene and shows the in-memory active event plus queued events.
+  Completed events are not retained as history. The AlertBox ticker uses the same
+  renderer/settings shape but shows only the active event.
+- **AlertBox ticker (announcement strip):** `AlertBoxTickerFrame` composes
   "Author — Kind · amount" and hands it to `Services/Compositor/AlertTickerRenderer.cs` —
   a strip rasterized **on the UI thread once per text** (cached, unpremultiplied
   straight-alpha pill) then positioned by pure byte-math per call, so the pump calls it
   from its own thread with no locking. **It is a dynamic overlay, NEVER baked** (the
   social bar IS baked into `BakeStaticBase`): `SceneCompositor.Render` +
   `CompositeLayers` take a trailing `tickerFrame` threaded from
-  `FramePump._alertTicker` (`Func<VideoFrame?>` ctor seam) through
+  `FramePump._alertTicker` (`Func<Scene, VideoFrame?>` ctor seam) through
   `RenderScene`/`RenderFull` and **mixed into `BuildFullRenderSignature`** so a scroll
-  changes the cache key. Toggled by `AlertShowTicker`.
+  changes the cache key. It reads the active scene's AlertBox settings.
   - **It draws INSIDE the Stream Alerts box (creator ruling 2026-09-26).** Was a global
     1920px bar pinned to the top edge: *"the ticker should appear over the stream alerts
     video, not over the entire preview window"*. The strip is now rendered at the
@@ -135,14 +143,16 @@ published decisions (recorded in `TASKS/task-43-native-alerts.md`):
     old fixed `140px/s` took ~17s per pass, so a 10s alert showed the message once.
     Same convention as the incumbent (Streamlabs: "Alert Duration" + "Text Delay", not a
     scroll-speed slider). Flash's off half returns null.
-  - **A ticker still needs its own preview element, not a `Source`.** It is a frame-pump
-    producer, so it never rides a per-element `Image`; `AlertOverlayLayer` takes a
-    `tickerPreviewSink` and publishes from `RefreshAlertPreviews()` (UI thread only —
-    the pump thread must never touch a `WriteableBitmap`); the VM writes it into one
-    reused `WriteableBitmap` (`AlertTickerImageSource`/`AlertTickerVisible`) bound to
-    `AlertTickerElement` in `PreviewPane.xaml`, mirroring `SocialBarElement`. The rect
-    is re-raised on **every** published frame, not just on a bitmap resize, so a box
-    moved or resized mid-alert tracks live.
+  - **AlertBox preview:** `tickerPreviewSink` publishes from
+    `RefreshAlertPreviews()` (UI thread only — the pump thread must never touch a
+    `WriteableBitmap`); the VM writes the box-sized frame into the reused bitmap bound
+    to `AlertTickerElement` in `PreviewPane.xaml`. The rect is re-raised on every
+    published frame so a box moved or resized mid-alert tracks live.
+  - **Standalone ticker preview/output:** it is an ordinary dynamic Source with a
+    per-element frame. `AlertOverlayLayer` updates each scene ticker's reusable
+    `VideoImageSource` on its UI timer for preview; the output resolver renders the
+    same source with that source's Ticker settings. It is not the AlertBox global
+    ticker callback, so each scene's layer and settings remain independent.
   - **Two measurement traps here:** a seamless marquee never goes blank (a wrapped copy
     enters as the pill clears), so count a pass by the pill's leading edge resetting, not
     by an empty frame; and the run is phase-started half a frame in, or the first