feat(notification-outbox): add daily terminal-row purge

src/ScadaLink.NotificationOutbox/Messages/InternalMessages.cs

@@ -52,4 +52,30 @@ internal static class InternalMessages
 
         private DispatchComplete() { }
     }
+
+    /// <summary>
+    /// Periodic tick that triggers a purge sweep of terminal notification rows. Started as a
+    /// periodic timer in <c>PreStart</c> at the configured <c>PurgeInterval</c>. A singleton
+    /// instance is reused so the timer carries no per-tick state.
+    /// </summary>
+    internal sealed class PurgeTick
+    {
+        /// <summary>The shared singleton tick instance scheduled by the purge timer.</summary>
+        internal static readonly PurgeTick Instance = new();
+
+        private PurgeTick() { }
+    }
+
+    /// <summary>
+    /// Completion signal for an asynchronous purge sweep, piped back to the actor so the
+    /// sweep's outcome (logged in the pipe projection) is observed on the actor thread.
+    /// Sent on both success and failure of the sweep.
+    /// </summary>
+    internal sealed class PurgeComplete
+    {
+        /// <summary>The shared singleton completion instance.</summary>
+        internal static readonly PurgeComplete Instance = new();
+
+        private PurgeComplete() { }
+    }
 }