perf: batch repository reads, index snapshots, centralize date encoding

- Add IEventRepository/IResultRepository.GetManyAsync to kill N+1 lookups at
  6 sites (backtest, outcome eval, both bet-journal paths, anomaly browsing,
  results selection); guarded by a Received(1).GetManyAsync test.
- Add EventRepository.QueryAsync to push date+sport filtering to SQL (was
  load-whole-range-then-filter); search/sort stay in-memory for Cyrillic order.
- Add AnomalyRepository.CountSinceAsync (unread badge) + ListByDateRangeAsync
  (feed date filter); add Event/Snapshot count methods for the dashboard.
- Add composite indexes IX_Snapshots_EventCode_CapturedAt and
  _EventCode_Source_CapturedAt via a new migration + model snapshot.
- Introduce SqliteDateText as the single source of the O-format date encoding
  shared by Mapping (read/write) and the repositories' range predicates.
- Fix LiveOddsPoller cadence drift (budget sleep against cycle time); make
  DetectAnomalies dedup O(1) per event; add Event.Title to dedup the title join.

Tests adapted to the batched GetManyAsync via a TestFixtures bridge.

tests/Marathon.UI.Tests/Pages/Results/ResultsLoaderTests.cs

@@ -32,6 +32,21 @@ public sealed class ResultsLoaderTests : MarathonTestContext
             sp.GetRequiredService<IEventRepository>(),
             sp.GetRequiredService<IResultRepository>(),
             NullLogger<PullResultsUseCase>.Instance));
+
+        // PullResultsUseCase batches selection-mode candidate resolution via
+        // GetManyAsync; route it through whatever GetAsync the test configures.
+        _eventRepo.GetManyAsync(Arg.Any<IReadOnlyCollection<EventId>>(), Arg.Any<CancellationToken>())
+            .Returns(ci =>
+            {
+                var ids = ci.Arg<IReadOnlyCollection<EventId>>();
+                var dict = new Dictionary<EventId, Event>();
+                foreach (var id in ids.Distinct())
+                {
+                    var ev = _eventRepo.GetAsync(id, CancellationToken.None).GetAwaiter().GetResult();
+                    if (ev is not null) dict[id] = ev;
+                }
+                return (IReadOnlyDictionary<EventId, Event>)dict;
+            });
     }
 
     [Fact]