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.Infrastructure/Migrations/20260505000000_InitialCreate.cs

@@ -1,3 +1,5 @@
+using Marathon.Infrastructure.Persistence;
+using Microsoft.EntityFrameworkCore.Infrastructure;
 using Microsoft.EntityFrameworkCore.Migrations;
 
 #nullable disable
@@ -5,6 +7,8 @@ using Microsoft.EntityFrameworkCore.Migrations;
 namespace Marathon.Infrastructure.Migrations;
 
 /// <inheritdoc />
+[DbContext(typeof(MarathonDbContext))]
+[Migration("20260505000000_InitialCreate")]
 public partial class InitialCreate : Migration
 {
     /// <inheritdoc />