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/UseCases/RunBacktestUseCase.cs

@@ -63,29 +63,16 @@ public sealed class RunBacktestUseCase
             return BacktestSimulator.Run(strategy, Array.Empty<BacktestCandidate>());
         }
 
-        // Distinct event lookups — minimises repo calls.
-        // TODO (perf, future): batch via IEventRepository.GetManyAsync /
-        // IResultRepository.GetManyAsync once those exist — currently shared
-        // with EvaluateAnomalyOutcomesUseCase, acceptable at expected volumes.
+        // Batched lookups — a single query each, replacing the prior per-event
+        // GetAsync round-trip (N+1 against SQLite).
         var distinctEventIds = anomalies.Select(a => a.EventId).Distinct().ToList();
 
-        var eventLookup = new Dictionary<DomainEventId, Event>(distinctEventIds.Count);
-        var resultLookup = new Dictionary<DomainEventId, EventResult>(distinctEventIds.Count);
-        var titles = new Dictionary<DomainEventId, string>(distinctEventIds.Count);
-        foreach (var id in distinctEventIds)
-        {
-            ct.ThrowIfCancellationRequested();
+        var eventLookup = await _events.GetManyAsync(distinctEventIds, ct).ConfigureAwait(false);
+        var resultLookup = await _results.GetManyAsync(distinctEventIds, ct).ConfigureAwait(false);
 
-            var ev = await _events.GetAsync(id, ct).ConfigureAwait(false);
-            if (ev is not null)
-            {
-                eventLookup[id] = ev;
-                titles[id] = string.Concat(ev.Side1Name, " vs ", ev.Side2Name);
-            }
-
-            var res = await _results.GetAsync(id, ct).ConfigureAwait(false);
-            if (res is not null) resultLookup[id] = res;
-        }
+        var titles = new Dictionary<DomainEventId, string>(eventLookup.Count);
+        foreach (var (id, ev) in eventLookup)
+            titles[id] = ev.Title;
 
         var candidates = new List<BacktestCandidate>(anomalies.Count);
         foreach (var anomaly in anomalies)