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.UI/Services/BetJournalService.cs

@@ -40,15 +40,15 @@ public sealed class BetJournalService : IBetJournalService
         if (report.Bets.Count == 0)
             return new BetJournalVm(report.Stats, Array.Empty<BetJournalRowVm>());
 
-        // Resolve event titles in one pass — distinct ids only.
+        // Resolve event titles in one batched query — distinct ids only (was N+1).
+        // Missing events (pruned by snapshot retention) fall back to the raw id.
         var distinctIds = report.Bets.Select(r => r.Bet.EventId).Distinct().ToList();
+        var events = await _events.GetManyAsync(distinctIds, ct).ConfigureAwait(false);
         var titles = new Dictionary<DomainEventId, string>(distinctIds.Count);
         foreach (var id in distinctIds)
         {
-            ct.ThrowIfCancellationRequested();
-            var ev = await _events.GetAsync(id, ct).ConfigureAwait(false);
-            titles[id] = ev is not null
-                ? string.Concat(ev.Side1Name, " vs ", ev.Side2Name)
+            titles[id] = events.TryGetValue(id, out var ev)
+                ? ev.Title
                 : id.Value;
         }