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/TestFixtures.cs

@@ -1,8 +1,10 @@
+using Marathon.Application.Abstractions;
 using Marathon.Application.Configuration;
 using Marathon.Domain.Entities;
 using Marathon.Domain.Enums;
 using Marathon.Domain.ValueObjects;
 using Microsoft.Extensions.Options;
+using NSubstitute;
 
 namespace Marathon.Application.Tests.UseCases;
 
@@ -13,6 +15,45 @@ internal static class TestFixtures
 {
     private static readonly TimeSpan MoscowOffset = TimeSpan.FromHours(3);
 
+    /// <summary>
+    /// Bridges the legacy per-id <c>GetAsync</c> stubs to the batched
+    /// <c>GetManyAsync</c> the use cases now call: each requested id is resolved
+    /// through whatever <c>GetAsync</c> was configured to return for it. Lets the
+    /// existing per-id <c>.Returns(...)</c> setups keep working unchanged.
+    /// </summary>
+    public static void BridgeGetMany(IEventRepository events)
+    {
+        events.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 = events.GetAsync(id, CancellationToken.None).GetAwaiter().GetResult();
+                    if (ev is not null) dict[id] = ev;
+                }
+                return (IReadOnlyDictionary<EventId, Event>)dict;
+            });
+    }
+
+    /// <inheritdoc cref="BridgeGetMany(IEventRepository)"/>
+    public static void BridgeGetMany(IResultRepository results)
+    {
+        results.GetManyAsync(Arg.Any<IReadOnlyCollection<EventId>>(), Arg.Any<CancellationToken>())
+            .Returns(ci =>
+            {
+                var ids = ci.Arg<IReadOnlyCollection<EventId>>();
+                var dict = new Dictionary<EventId, EventResult>();
+                foreach (var id in ids.Distinct())
+                {
+                    var r = results.GetAsync(id, CancellationToken.None).GetAwaiter().GetResult();
+                    if (r is not null) dict[id] = r;
+                }
+                return (IReadOnlyDictionary<EventId, EventResult>)dict;
+            });
+    }
+
     /// <summary>Creates a minimal valid <see cref="Event"/> with the given event ID string.</summary>
     public static Event MakeEvent(string eventIdValue = "12345678")
     {