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.

src/Marathon.Application/Abstractions/IAnomalyRepository.cs

@@ -5,4 +5,22 @@ namespace Marathon.Application.Abstractions;
 /// <summary>
 /// Repository for <see cref="Anomaly"/> domain entities.
 /// </summary>
-public interface IAnomalyRepository : IRepository<Guid, Anomaly>;
+public interface IAnomalyRepository : IRepository<Guid, Anomaly>
+{
+    /// <summary>
+    /// Server-side count of anomalies detected strictly after <paramref name="since"/>.
+    /// Backs the unread badge without materialising the table.
+    /// </summary>
+    Task<int> CountSinceAsync(DateTimeOffset since, CancellationToken ct = default);
+
+    /// <summary>
+    /// Anomalies whose <see cref="Anomaly.DetectedAt"/> falls in the inclusive
+    /// [<paramref name="from"/>..<paramref name="to"/>] window (either bound may be
+    /// null for open-ended), ordered newest-first. Pushes the temporal filter to SQL;
+    /// severity / sport filtering remains a service concern (needs the event join).
+    /// </summary>
+    Task<IReadOnlyList<Anomaly>> ListByDateRangeAsync(
+        DateTimeOffset? from,
+        DateTimeOffset? to,
+        CancellationToken ct = default);
+}