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.Application/Services/PostgresqlHistorySnapshotReader.cs

@@ -0,0 +1,19 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using QuantEngine.Core.Interfaces;
+
+namespace QuantEngine.Application.Services
+{
+    public class PostgresqlHistorySnapshotReader : IPostgresqlHistorySnapshotReader
+    {
+        private readonly IPostgresqlHistoryStore _store;
+
+        public PostgresqlHistorySnapshotReader(IPostgresqlHistoryStore store)
+        {
+            _store = store;
+        }
+
+        public Task<IReadOnlyList<IDictionary<string, object?>>> ReadAsync(string domain, int limit = 500)
+            => _store.SnapshotAsync(domain, limit);
+    }
+}