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.Infrastructure/Migrations/MarathonDbContextModelSnapshot.cs

@@ -92,6 +92,8 @@ partial class MarathonDbContextModelSnapshot : ModelSnapshot
             b.Property<int>("Source").HasColumnType("INTEGER");
             b.HasKey("Id");
             b.HasIndex("EventCode").HasDatabaseName("IX_Snapshots_EventCode");
+            b.HasIndex("EventCode", "CapturedAt").HasDatabaseName("IX_Snapshots_EventCode_CapturedAt");
+            b.HasIndex("EventCode", "Source", "CapturedAt").HasDatabaseName("IX_Snapshots_EventCode_Source_CapturedAt");
             b.ToTable("Snapshots");
         });