feat(notification-outbox): add dispatcher loop to NotificationOutboxActor

src/ScadaLink.NotificationOutbox/Messages/InternalMessages.cs

@@ -26,4 +26,30 @@ internal static class InternalMessages
         IActorRef Sender,
         bool Succeeded,
         string? Error);
+
+    /// <summary>
+    /// Periodic tick that triggers a dispatch sweep. Started as a periodic timer in
+    /// <c>PreStart</c> at the configured <c>DispatchInterval</c>. A singleton instance is
+    /// reused so the timer carries no per-tick state.
+    /// </summary>
+    internal sealed class DispatchTick
+    {
+        /// <summary>The shared singleton tick instance scheduled by the dispatch timer.</summary>
+        internal static readonly DispatchTick Instance = new();
+
+        private DispatchTick() { }
+    }
+
+    /// <summary>
+    /// Completion signal for an asynchronous dispatch sweep, piped back to the actor so the
+    /// in-flight guard is cleared on the actor thread. Sent on both success and failure of
+    /// the sweep — the actor only needs to know the sweep has finished.
+    /// </summary>
+    internal sealed class DispatchComplete
+    {
+        /// <summary>The shared singleton completion instance.</summary>
+        internal static readonly DispatchComplete Instance = new();
+
+        private DispatchComplete() { }
+    }
 }