perf(notification-outbox): bounded parallel delivery within a dispatch sweep (default 4)

src/ZB.MOM.WW.ScadaBridge.NotificationOutbox/NotificationOutboxOptions.cs

@@ -48,4 +48,23 @@ public class NotificationOutboxOptions
     /// Default: 1 minute.
     /// </summary>
     public TimeSpan DeliveredKpiWindow { get; set; } = TimeSpan.FromMinutes(1);
+
+    /// <summary>
+    /// Maximum number of notifications delivered concurrently within a single dispatch
+    /// sweep. Delivery is I/O-bound (SMTP / Twilio round trips of seconds each), so a
+    /// purely sequential sweep caps throughput at ~1/attempt-latency regardless of batch
+    /// size — an alarm storm queued behind one slow SMTP host would drain far too slowly
+    /// (arch-review 04). Each concurrent delivery runs on its own DI scope/repository, so
+    /// there is no shared-DbContext contention. Set to <c>1</c> to preserve the strictly
+    /// sequential behaviour. Clamped to a minimum of 1 via <see cref="ResolvedMaxParallelDeliveries"/>.
+    /// Default: 4.
+    /// </summary>
+    public int MaxParallelDeliveries { get; set; } = 4;
+
+    /// <summary>
+    /// Resolves the effective delivery parallelism, clamped to at least 1 so a
+    /// misconfigured <c>0</c>/negative value cannot stall the dispatcher (a zero-count
+    /// semaphore would deadlock the sweep).
+    /// </summary>
+    public int ResolvedMaxParallelDeliveries => MaxParallelDeliveries < 1 ? 1 : MaxParallelDeliveries;
 }