feat: postgres history-first 계약과 적재 경로 추가

- PostgreSQL history contract와 schema/validator를 추가했습니다.
- .NET history store, snapshot reader, repository, migration을 연결했습니다.
- history-first 운영 모델 문서와 daily signal tracking 문구를 정리했습니다.

src/dotnet/QuantEngine.Core/Interfaces/IPostgresqlHistorySnapshotReader.cs

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+using System.Collections.Generic;
+using System.Threading.Tasks;
+
+namespace QuantEngine.Core.Interfaces
+{
+    public interface IPostgresqlHistorySnapshotReader
+    {
+        Task<IReadOnlyList<IDictionary<string, object?>>> ReadAsync(string domain, int limit = 500);
+    }
+}

src/dotnet/QuantEngine.Core/Interfaces/IPostgresqlHistoryStore.cs

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+using System.Collections.Generic;
+using System.Threading.Tasks;
+
+namespace QuantEngine.Core.Interfaces
+{
+    public interface IPostgresqlHistoryStore
+    {
+        Task<int> AppendAsync(string domain, IDictionary<string, object?> payload);
+        Task<IReadOnlyList<IDictionary<string, object?>>> SnapshotAsync(string domain, int limit = 500);
+    }
+}

src/dotnet/QuantEngine.Core/Models/HistoryRow.cs

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+using System.Collections.Generic;
+
+namespace QuantEngine.Core.Models
+{
+    public class HistoryRow
+    {
+        public string Domain { get; set; } = string.Empty;
+        public IDictionary<string, object?> Payload { get; set; } = new Dictionary<string, object?>();
+    }
+}