perf(scraping): parallel HTTP fan-out, sequential DB persist (HIGH)

The Pull*UseCase implementations issued one HTTP request at a time despite
Scraping:MaxConcurrentRequests=4. With 30–80 live events and ~1s per
fetch, a 5–10s live cadence target was unreachable; cycles overflowed
the configured interval.

* New Marathon.Application.Configuration.ScrapingThrottle bound from the
  shared Scraping:* section. Exposes only MaxConcurrentRequests so the
  Application layer doesn't pull in the Infrastructure-side ScrapingOptions.
* PullLiveOddsUseCase + PullUpcomingEventsUseCase split into two phases:
  - Phase 1 — Parallel.ForEachAsync over the event list with
    MaxDegreeOfParallelism = throttle.MaxConcurrentRequests. The scraper's
    Polly rate limiter still throttles to RequestsPerSecond underneath
    this fan-out, so spikes are smoothed before they hit the bookmaker.
  - Phase 2 — sequential foreach over the (Event, Snapshot) tuples
    captured in Phase 1, doing event upsert + snapshot insert. EF Core
    DbContext is not thread-safe so all DB writes stay on a single thread.
* InfrastructureModule binds ScrapingThrottle alongside AnomalyOptions.
* Failed snapshot scrapes in Phase 1 mean the event row is also NOT
  persisted in Phase 2 — previously we'd persist the row even when the
  snapshot scrape failed, leaving an orphan event with no odds. Updated
  the regression test accordingly.
* Test fixture exposes TestFixtures.Throttle(maxConcurrentRequests=1) for
  deterministic sequential test runs.
* One existing NSubstitute setup that chained Arg.Is<>() across two
  configurations was rewritten to use a single Arg.Any<>() with inline
  branching — chained matchers were leaking and returning wrong results.

tests/Marathon.Infrastructure.Tests/Workers/LiveOddsPollerTests.cs

@@ -1,5 +1,6 @@
 using FluentAssertions;
 using Marathon.Application.Abstractions;
+using Marathon.Application.Configuration;
 using Marathon.Application.UseCases;
 using Marathon.Domain.Entities;
 using Marathon.Domain.Enums;
@@ -47,11 +48,19 @@ public sealed class LiveOddsPollerTests
                 sp.GetRequiredService<IOddsScraper>(),
                 sp.GetRequiredService<IEventRepository>(),
                 sp.GetRequiredService<ISnapshotRepository>(),
+                StaticThrottle(),
                 NullLogger<PullLiveOddsUseCase>.Instance));
 
         return services.BuildServiceProvider();
     }
 
+    private static IOptionsMonitor<ScrapingThrottle> StaticThrottle()
+    {
+        var monitor = Substitute.For<IOptionsMonitor<ScrapingThrottle>>();
+        monitor.CurrentValue.Returns(new ScrapingThrottle { MaxConcurrentRequests = 1 });
+        return monitor;
+    }
+
     private static IOptionsMonitor<WorkerOptions> BuildOptions(
         bool enabled = true,
         int intervalSeconds = 0)