feat(historian): SQL Server retention purge (RetentionDays, Primary-gated, applock) (#667)

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Historian/IHistorianRetention.cs

@@ -0,0 +1,25 @@
+namespace ZB.MOM.WW.OtOpcUa.Runtime.Historian;
+
+/// <summary>Rows removed from one historian table by a single purge pass.</summary>
+/// <param name="Table">Unqualified table name (the <c>table</c> tag on the retention counter).</param>
+/// <param name="Rows">Rows deleted.</param>
+public sealed record RetentionPurgeResult(string Table, long Rows);
+
+/// <summary>
+///     Backend-side retention purge: deletes history older than the backend's configured retention window.
+///     Created by <see cref="IHistorianBackendProvider.CreateRetention"/> and driven on a timer by
+///     <see cref="HistorianRetentionService"/>, which owns scheduling and the Primary gate.
+/// </summary>
+public interface IHistorianRetention
+{
+    /// <summary>How often <see cref="HistorianRetentionService"/> should run a pass.</summary>
+    TimeSpan Interval { get; }
+
+    /// <summary>
+    ///     Runs one purge pass. Returns an empty list when the pass was skipped (another process holds the
+    ///     purge lock). Implementations honour cancellation between batches.
+    /// </summary>
+    /// <param name="ct">Cancellation.</param>
+    /// <returns>Per-table deleted-row counts.</returns>
+    Task<IReadOnlyList<RetentionPurgeResult>> PurgeAsync(CancellationToken ct);
+}