feat(notification-outbox): add notification message and outbox query contracts

src/ScadaLink.Commons/Messages/Notification/NotificationMessages.cs

@@ -0,0 +1,42 @@
+namespace ScadaLink.Commons.Messages.Notification;
+
+/// <summary>
+/// Site -> Central: submit a notification for central delivery.
+/// Fire-and-forget with ack; the site retries until a <see cref="NotificationSubmitAck"/> is received.
+/// </summary>
+public record NotificationSubmit(
+    string NotificationId,
+    string ListName,
+    string Subject,
+    string Body,
+    string SourceSiteId,
+    string? SourceInstanceId,
+    string? SourceScript,
+    DateTimeOffset SiteEnqueuedAt);
+
+/// <summary>
+/// Central -> Site: ack sent after the notification row is persisted.
+/// Idempotent — safe to re-send for the same <see cref="NotificationId"/>.
+/// </summary>
+public record NotificationSubmitAck(
+    string NotificationId,
+    bool Accepted,
+    string? Error);
+
+/// <summary>
+/// Site -> Central: query the central delivery status for a <see cref="NotificationId"/>.
+/// </summary>
+public record NotificationStatusQuery(
+    string CorrelationId,
+    string NotificationId);
+
+/// <summary>
+/// Central -> Site: response carrying the current delivery status for a queried notification.
+/// </summary>
+public record NotificationStatusResponse(
+    string CorrelationId,
+    bool Found,
+    string Status,
+    int RetryCount,
+    string? LastError,
+    DateTimeOffset? DeliveredAt);