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.Application.Tests/UseCases/RunBacktestUseCaseTests.cs

@@ -25,6 +25,13 @@ public sealed class RunBacktestUseCaseTests
     private readonly IEventRepository _events = Substitute.For<IEventRepository>();
     private readonly IResultRepository _results = Substitute.For<IResultRepository>();
 
+    public RunBacktestUseCaseTests()
+    {
+        // Use case batches event/result loads via GetManyAsync; route through per-id stubs.
+        TestFixtures.BridgeGetMany(_events);
+        TestFixtures.BridgeGetMany(_results);
+    }
+
     private RunBacktestUseCase CreateSut() =>
         new(_anomalies, _events, _results, NullLogger<RunBacktestUseCase>.Instance);