fix(persistence): drop broken Guid lookup on ISnapshotRepository (CRITICAL)

Snapshots are append-only and identified by the composite (EventId, CapturedAt),
not a surrogate Guid. The previous implementation inherited
IRepository<Guid, OddsSnapshot> and faked the lookup with (long)key.GetHashCode()
on a long auto-increment PK — collision-prone and non-portable. Nobody called
GetAsync(Guid) / DeleteAsync(Guid) anyway.

* ISnapshotRepository no longer extends IRepository<Guid, OddsSnapshot>; it
  exposes only the methods snapshots actually have: ListAsync, ListByEventAsync,
  AddAsync, SaveChangesAsync.
* SnapshotRepository drops the broken Get/Update/Delete methods.

src/Marathon.Application/Abstractions/ISnapshotRepository.cs

@@ -6,11 +6,23 @@ namespace Marathon.Application.Abstractions;
 /// <summary>
 /// Repository for <see cref="OddsSnapshot"/> domain entities.
 /// </summary>
-public interface ISnapshotRepository : IRepository<Guid, OddsSnapshot>
+/// <remarks>
+/// Snapshots are append-only and identified by the composite (EventId, CapturedAt)
+/// rather than a surrogate key, so this contract intentionally does NOT extend
+/// <see cref="IRepository{TKey, TEntity}"/> — point lookup by Guid would be
+/// meaningless. Use <see cref="ListByEventAsync"/> for retrieval.
+/// </remarks>
+public interface ISnapshotRepository
 {
+    Task<IReadOnlyList<OddsSnapshot>> ListAsync(CancellationToken ct = default);
+
     Task<IReadOnlyList<OddsSnapshot>> ListByEventAsync(
         EventId eventId,
         DateTimeOffset from,
         DateTimeOffset to,
         CancellationToken ct = default);
+
+    Task AddAsync(OddsSnapshot entity, CancellationToken ct = default);
+
+    Task SaveChangesAsync(CancellationToken ct = default);
 }