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.Application.Tests/UseCases/PullLiveOddsUseCaseTests.cs

@@ -18,6 +18,7 @@ public sealed class PullLiveOddsUseCaseTests
 
     private PullLiveOddsUseCase CreateSut() =>
         new(_scraper, _eventRepo, _snapshotRepo,
+            TestFixtures.Throttle(),
             NullLogger<PullLiveOddsUseCase>.Instance);
 
     [Fact]

tests/Marathon.Application.Tests/UseCases/PullUpcomingEventsUseCaseTests.cs

@@ -18,6 +18,7 @@ public sealed class PullUpcomingEventsUseCaseTests
 
     private PullUpcomingEventsUseCase CreateSut() =>
         new(_scraper, _eventRepo, _snapshotRepo,
+            TestFixtures.Throttle(),
             NullLogger<PullUpcomingEventsUseCase>.Instance);
 
     [Fact]
@@ -88,24 +89,30 @@ public sealed class PullUpcomingEventsUseCaseTests
         var ev2 = TestFixtures.MakeEvent("22222222");
         var events = new List<Event> { ev1, ev2 }.AsReadOnly();
 
-        _scraper.ScrapeUpcomingAsync(null, Arg.Any<CancellationToken>()).Returns(events);
+        _scraper.ScrapeUpcomingAsync(Arg.Any<SportCode?>(), Arg.Any<CancellationToken>()).Returns(events);
         _eventRepo.GetAsync(Arg.Any<EventId>(), Arg.Any<CancellationToken>()).Returns((Event?)null);
 
+        // Inline routing of the throwing-vs-passing branch on event id, since chained
+        // Arg.Is<>() setups can leak matchers across NSubstitute invocations.
         _scraper.ScrapeEventOddsAsync(
-                Arg.Is<Event>(e => e.Id == ev1.Id), OddsSource.PreMatch, Arg.Any<CancellationToken>())
-            .ThrowsAsync(new HttpRequestException("site down"));
-        _scraper.ScrapeEventOddsAsync(
-                Arg.Is<Event>(e => e.Id == ev2.Id), OddsSource.PreMatch, Arg.Any<CancellationToken>())
-            .Returns(TestFixtures.MakeSnapshot(ev2.Id));
+                Arg.Any<Event>(), OddsSource.PreMatch, Arg.Any<CancellationToken>())
+            .Returns<OddsSnapshot>(ci =>
+            {
+                var evArg = ci.Arg<Event>();
+                if (evArg.Id == ev1.Id)
+                    throw new HttpRequestException("site down");
+                return TestFixtures.MakeSnapshot(evArg.Id);
+            });
 
         var sut = CreateSut();
 
         // Act — should not throw
         var (processed, newEvents, snapshots) = await sut.ExecuteAsync(CancellationToken.None);
 
-        // Assert
+        // Assert: ev1's snapshot scrape failed in Phase 1, so it's not even
+        // attempted in Phase 2 — no orphan event row gets persisted.
         processed.Should().Be(2);
-        newEvents.Should().Be(2);
+        newEvents.Should().Be(1, "ev1's snapshot failed so it was not persisted");
         snapshots.Should().Be(1, "only ev2 snapshot succeeded");
     }
 

tests/Marathon.Application.Tests/UseCases/TestFixtures.cs

@@ -1,6 +1,8 @@
+using Marathon.Application.Configuration;
 using Marathon.Domain.Entities;
 using Marathon.Domain.Enums;
 using Marathon.Domain.ValueObjects;
+using Microsoft.Extensions.Options;
 
 namespace Marathon.Application.Tests.UseCases;
 
@@ -42,4 +44,23 @@ internal static class TestFixtures
     {
         return new EventResult(eventId, 2, 1, Side.Side1, DateTimeOffset.UtcNow);
     }
+
+    /// <summary>
+    /// Creates an <see cref="IOptionsMonitor{TOptions}"/> that always returns the given
+    /// throttle. Use 1 for sequential test behaviour, higher values to exercise fan-out.
+    /// </summary>
+    public static IOptionsMonitor<ScrapingThrottle> Throttle(int maxConcurrentRequests = 1) =>
+        new StaticOptionsMonitor<ScrapingThrottle>(new ScrapingThrottle
+        {
+            MaxConcurrentRequests = maxConcurrentRequests,
+        });
+
+    private sealed class StaticOptionsMonitor<T> : IOptionsMonitor<T> where T : class
+    {
+        private readonly T _value;
+        public StaticOptionsMonitor(T value) => _value = value;
+        public T CurrentValue => _value;
+        public T Get(string? name) => _value;
+        public IDisposable? OnChange(Action<T, string?> listener) => null;
+    }
 }

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)