harden(notifications): per-item marker advance + Telegram client timeout

Review follow-ups: advance the dispatcher's "since" marker after each delivered
alert (not once per batch) so a future throwing sink can't re-deliver already-sent
alerts; give the Telegram HttpClient a 15s timeout so a hung connection can't stall
the dispatch loop.

src/Marathon.Infrastructure/InfrastructureModule.cs

@@ -64,7 +64,8 @@ public static class InfrastructureModule
         // Outbound anomaly notifications (Telegram). Sink + dispatcher are always
         // registered; the dispatcher idles until Notifications:Enabled is true and
         // the sink no-ops until a bot token + chat id are configured.
-        services.AddHttpClient(TelegramNotificationSink.HttpClientName);
+        services.AddHttpClient(TelegramNotificationSink.HttpClientName, client =>
+            client.Timeout = TimeSpan.FromSeconds(15));
         services.AddSingleton<INotificationSink, TelegramNotificationSink>();
         services.AddHostedService<AnomalyNotificationDispatcher>();
 

src/Marathon.Infrastructure/Workers/AnomalyNotificationDispatcher.cs

@@ -62,18 +62,22 @@ internal sealed class AnomalyNotificationDispatcher : BackgroundService
                 var useCase = scope.ServiceProvider.GetRequiredService<GetPendingAnomalyNotificationsUseCase>();
                 var pending = await useCase.ExecuteAsync(_since, opts.MinScore, stoppingToken);
 
+                var dispatched = 0;
                 foreach (var notification in pending)
                 {
                     stoppingToken.ThrowIfCancellationRequested();
                     await _sink.SendAsync(notification, stoppingToken);
+                    // Advance the marker per delivered item (pending is oldest-first) so that
+                    // if a future sink ever threw mid-batch, the already-sent alerts are not
+                    // re-delivered on the next cycle — only the unsent tail is retried.
+                    _since = notification.DetectedAt.AddTicks(1);
+                    dispatched++;
                 }
 
-                if (pending.Count > 0)
+                if (dispatched > 0)
                 {
-                    // pending is oldest-first; advance strictly past the newest sent.
-                    _since = pending[^1].DetectedAt.AddTicks(1);
                     _logger.LogInformation(
-                        "AnomalyNotificationDispatcher: dispatched {Count} alert(s)", pending.Count);
+                        "AnomalyNotificationDispatcher: dispatched {Count} alert(s)", dispatched);
                 }
             }
             catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested)