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);
 }

src/Marathon.Infrastructure/Persistence/Repositories/SnapshotRepository.cs

@@ -11,18 +11,6 @@ internal sealed class SnapshotRepository : ISnapshotRepository
 
     public SnapshotRepository(MarathonDbContext db) => _db = db;
 
-    public async Task<OddsSnapshot?> GetAsync(Guid key, CancellationToken ct = default)
-    {
-        var entity = await _db.Snapshots
-            .Include(s => s.Bets)
-            .FirstOrDefaultAsync(s => s.Id == (long)key.GetHashCode(), ct);
-        // Note: Guid→long mapping is lossy for GetAsync by Guid; the repo interface requires Guid key.
-        // Snapshots are typically retrieved by event, not directly by id.
-        // A proper implementation would store the Guid as a TEXT column.
-        // For now, this method is functionally available — callers prefer ListByEventAsync.
-        return entity is null ? null : Mapping.ToDomain(entity);
-    }
-
     public async Task<IReadOnlyList<OddsSnapshot>> ListAsync(CancellationToken ct = default)
     {
         var entities = await _db.Snapshots.AsNoTracking()
@@ -56,21 +44,6 @@ internal sealed class SnapshotRepository : ISnapshotRepository
         await _db.Snapshots.AddAsync(efEntity, ct);
     }
 
-    public Task UpdateAsync(OddsSnapshot entity, CancellationToken ct = default)
-    {
-        // Snapshots are immutable once written — update is not a typical operation.
-        var efEntity = Mapping.ToEntity(entity);
-        _db.Snapshots.Update(efEntity);
-        return Task.CompletedTask;
-    }
-
-    public async Task DeleteAsync(Guid key, CancellationToken ct = default)
-    {
-        var entity = await _db.Snapshots.FindAsync([(long)key.GetHashCode()], ct);
-        if (entity is not null)
-            _db.Snapshots.Remove(entity);
-    }
-
     public async Task SaveChangesAsync(CancellationToken ct = default) =>
         await _db.SaveChangesAsync(ct);
 }