feat: BRB backdrop seed + docs refresh + Polar org ID

- Add brb-backdrop.jpg asset, SeedBrbBackdrop() mirrors Starting pattern
- Update HANDOFF.md (was stale), TASKS.md (TASK 10 ready, TASK 15 2/5), ai.md (org ID)
- Polar org ID c05fb364 obtained via OAT, recorded in CREDENTIALS.md

ViewModels/MainViewModel.cs

@@ -1356,6 +1356,7 @@ public class MainViewModel : ViewModelBase
                     AddScene(name);
             }
             SeedStartingBackdrop();
+            SeedBrbBackdrop();
 
             // The live backdrop belongs to Live only; a pre-policy DB may have
             // backdrops lingering in other scenes — drop them, then
@@ -1764,6 +1765,34 @@ public class MainViewModel : ViewModelBase
         catch { /* non-critical — stream works fine without a default bg */ }
     }
 
+    private void SeedBrbBackdrop()
+    {
+        try
+        {
+            var brb = Scenes.FirstOrDefault(s => SceneCatalog.Is(s.Name, "BRB"));
+            if (brb == null) return;
+            if (brb.Elements.OfType<Source>().Any(s => s.Type == SourceType.Background)) return;
+
+            var uri = new Uri("pack://application:,,,/Assets/brb-backdrop.jpg", UriKind.Absolute);
+            using var stream = Application.GetResourceStream(uri)?.Stream;
+            if (stream == null) return;
+            using var ms = new MemoryStream();
+            stream.CopyTo(ms);
+            var bytes = ms.ToArray();
+            var assetId = AddAsset(bytes);
+            if (assetId == null) return;
+            brb.Elements.Add(new Source
+            {
+                Name = "Background",
+                Type = SourceType.Background,
+                AssetId = assetId,
+                Width = 1920,
+                Height = 1080,
+            });
+        }
+        catch { /* non-critical — stream works fine without a default bg */ }
+    }
+
     private void BeginEditElement(SceneElement? element)
     {
         if (element == null) return;