feat(phase-4): application layer + background workers — 202/202 tests green

Use cases (Marathon.Application/UseCases/):
- PullUpcomingEventsUseCase: scrape + persist new events + capture pre-match snapshots
- PullLiveOddsUseCase: refresh live snapshots for all stored events
- PullResultsUseCase: Phase 4 scaffold; delegates to ScrapeResultsAsync (Phase 3 no-op);
  Phase 8 will replace with watch-list polling
- ExportToExcelUseCase: resolves export dir from StorageOptions, delegates to IExcelExporter

ApplicationModule.AddMarathonApplication(IServiceCollection) — no IConfiguration needed.

Background workers (Marathon.Infrastructure/Workers/):
- UpcomingEventsPoller: Cronos 6-field cron schedule (default every 6 h)
- LiveOddsPoller: fixed interval (WorkerOptions.LivePollIntervalSeconds, default 30 s)
- ResultsWatchListPoller: scaffold, disabled by default (WorkerOptions.ResultsPollerEnabled=false)
All three: exception-swallowing, cancellation-aware, scoped DI via CreateAsyncScope().

InfrastructureModule.AddMarathonInfrastructure(IServiceCollection, IConfiguration):
- Composes AddMarathonPersistence + AddMarathonScraping + WorkerOptions + 3 hosted services

App.xaml.cs: replace reflection-based TryAddApplicationAndInfrastructure with direct
AddMarathonApplication() + AddMarathonInfrastructure(config) calls.

Resolved Phase 3 TODO: bind Sports:Basketball:QuarterMode from config in ScrapingModule.

appsettings.json: add Workers.LivePollIntervalSeconds, ResultsPollIntervalSeconds,
ResultsPollerEnabled; add Sports.Basketball.QuarterMode.

Settings.razor + WorkerOptions (UI) + SharedResource.*.resx: surface new Workers fields.

Tests: +14 Application use-case tests, +3 Infrastructure worker tests (185 → 202 total).

src/Marathon.Application/ApplicationModule.cs

@@ -0,0 +1,35 @@
+using Marathon.Application.UseCases;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Marathon.Application;
+
+/// <summary>
+/// DI registration helpers for the Marathon.Application layer.
+/// Call <see cref="AddMarathonApplication"/> from the composition root (host or
+/// <c>InfrastructureModule</c>).
+/// </summary>
+public static class ApplicationModule
+{
+    /// <summary>
+    /// Registers all Application-layer use cases with <c>Scoped</c> lifetime.
+    /// Use cases are scoped so that each background-service cycle or UI request
+    /// gets a fresh unit-of-work from its own DI scope.
+    /// </summary>
+    /// <remarks>
+    /// No <see cref="Microsoft.Extensions.Configuration.IConfiguration"/> is
+    /// required here — the Application layer has no direct configuration bindings.
+    /// Infrastructure and UI layers bind their own options against the shared
+    /// JSON sections.
+    /// </remarks>
+    public static IServiceCollection AddMarathonApplication(this IServiceCollection services)
+    {
+        ArgumentNullException.ThrowIfNull(services);
+
+        services.AddScoped<PullUpcomingEventsUseCase>();
+        services.AddScoped<PullLiveOddsUseCase>();
+        services.AddScoped<PullResultsUseCase>();
+        services.AddScoped<ExportToExcelUseCase>();
+
+        return services;
+    }
+}