fix(host): wire DB migration init + Plotly CDN + attribute fix on hand-written migration

Three fixes surfaced when launching the WPF host for the first time:

1. App.xaml.cs — call MarathonDbContextInitializer.InitializeAsync()
   between Host.Build() and Host.Start() so EF migrations + WAL pragma
   are applied BEFORE BackgroundServices race to query the DB. Without
   this, all pollers crashed on 'no such table: Events'.

2. wwwroot/index.html — added <script src='https://cdn.plot.ly/plotly-2.35.2.min.js'>
   before blazor.webview.js. Phase 6 reviewer flagged this for Phase 9,
   but charts are unrenderable without it; better to ship now.

3. Migrations/20260505000000_InitialCreate.cs — added [DbContext] and
   [Migration('20260505000000_InitialCreate')] attributes. Phase 2's
   hand-written migration was missing both, so EF saw 'no migrations to
   apply' even on a fresh DB. With the attributes, the migration runs
   on first launch and creates all tables (Events, Snapshots, Bets,
   EventResults, Anomalies, Sports, Leagues).

Verified: clean DB → migration applied → all 7 tables created → pollers
run with empty results (no data yet — UpcomingEventsPoller fires every 6h
by default; first scrape will populate the DB).

src/Marathon.Hosts.WpfBlazor/App.xaml.cs

@@ -83,6 +83,17 @@ public partial class App : System.Windows.Application
         builder.Services.AddSingleton<MainWindow>();
 
         Host = builder.Build();
+
+        // Apply EF migrations + WAL pragma BEFORE Host.Start() so the BackgroundServices
+        // (LiveOddsPoller, AnomalyDetectionPoller, etc.) don't race the DB schema creation.
+        // Resolved in a scope because MarathonDbContextInitializer is Scoped (DbContext lifetime).
+        using (var initScope = Host.Services.CreateScope())
+        {
+            var initializer = initScope.ServiceProvider
+                .GetRequiredService<Marathon.Infrastructure.Persistence.MarathonDbContextInitializer>();
+            initializer.InitializeAsync().GetAwaiter().GetResult();
+        }
+
         Host.Start();
 
         // Apply default culture from configuration before any UI renders.