feat(notification-outbox): add NotificationOutbox repository

src/ScadaLink.Commons/Types/Notifications/NotificationKpiSnapshot.cs

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+namespace ScadaLink.Commons.Types.Notifications;
+
+/// <summary>
+/// Point-in-time operational metrics for the central notification outbox,
+/// surfaced on the health dashboard.
+/// </summary>
+/// <param name="QueueDepth">Count of non-terminal rows (Pending + Retrying).</param>
+/// <param name="StuckCount">
+/// Count of non-terminal rows (Pending/Retrying) whose <c>CreatedAt</c> is older
+/// than the supplied stuck cutoff.
+/// </param>
+/// <param name="ParkedCount">Count of rows in the Parked status.</param>
+/// <param name="DeliveredLastInterval">
+/// Count of Delivered rows whose <c>DeliveredAt</c> is at or after the supplied
+/// "delivered since" timestamp.
+/// </param>
+/// <param name="OldestPendingAge">
+/// Age of the oldest non-terminal row (<c>now - min(CreatedAt)</c>), or <c>null</c>
+/// when there are no non-terminal rows.
+/// </param>
+public record NotificationKpiSnapshot(
+    int QueueDepth,
+    int StuckCount,
+    int ParkedCount,
+    int DeliveredLastInterval,
+    TimeSpan? OldestPendingAge);

src/ScadaLink.Commons/Types/Notifications/NotificationOutboxFilter.cs

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+using ScadaLink.Commons.Types.Enums;
+
+namespace ScadaLink.Commons.Types.Notifications;
+
+/// <summary>
+/// Query filter for the central notification outbox. All members are optional;
+/// an unset member means "no constraint on that dimension".
+/// </summary>
+/// <param name="Status">Restrict to a single lifecycle status.</param>
+/// <param name="Type">Restrict to a single delivery channel.</param>
+/// <param name="SourceSiteId">Restrict to notifications originating at a given site.</param>
+/// <param name="ListName">Restrict to a single notification list.</param>
+/// <param name="SubjectKeyword">Substring matched against <c>Subject</c>.</param>
+/// <param name="StuckOnly">
+/// When <c>true</c>, restrict to non-terminal rows (Pending/Retrying) whose
+/// <c>CreatedAt</c> is older than <see cref="StuckCutoff"/>.
+/// </param>
+/// <param name="StuckCutoff">Rows with <c>CreatedAt</c> older than this count as stuck.</param>
+/// <param name="From">Inclusive lower bound on <c>CreatedAt</c>.</param>
+/// <param name="To">Inclusive upper bound on <c>CreatedAt</c>.</param>
+public record NotificationOutboxFilter(
+    NotificationStatus? Status = null,
+    NotificationType? Type = null,
+    string? SourceSiteId = null,
+    string? ListName = null,
+    string? SubjectKeyword = null,
+    bool StuckOnly = false,
+    DateTimeOffset? StuckCutoff = null,
+    DateTimeOffset? From = null,
+    DateTimeOffset? To = null);