feat(notifications): config-gated Telegram anomaly alerts

- INotificationSink + AnomalyNotification (Application) and a testable
  GetPendingAnomalyNotificationsUseCase (date+score filter, event-title join,
  oldest-first). 4 use-case tests.
- TelegramNotificationSink posts to the Bot API via HttpClient (no SDK dependency);
  no-ops with a warning when unconfigured and never logs the token.
- AnomalyNotificationDispatcher BackgroundService: startup-baselined marker advanced
  past the newest sent (gap- and dup-free); idles until Notifications:Enabled.
- Wire options + named client + sink + dispatcher in InfrastructureModule. Add a
  secret-free Notifications section + steam-move tuning to appsettings.json
  (bot token + chat id go in appsettings.Local.json only).

src/Marathon.Application/UseCases/GetPendingAnomalyNotificationsUseCase.cs

@@ -0,0 +1,62 @@
+using Marathon.Application.Abstractions;
+using Microsoft.Extensions.Logging;
+
+namespace Marathon.Application.UseCases;
+
+/// <summary>
+/// Shapes the anomalies worth alerting on: those detected at or after a caller-supplied
+/// marker whose score clears a minimum, joined with their event titles. Pure of any
+/// transport concern — the dispatcher decides cadence and the sink decides delivery.
+/// </summary>
+/// <remarks>
+/// Results are ordered oldest-first so the caller can advance its "since" marker to the
+/// last item's <see cref="AnomalyNotification.DetectedAt"/> (plus one tick) without gaps
+/// or duplicates.
+/// </remarks>
+public sealed class GetPendingAnomalyNotificationsUseCase
+{
+    private readonly IAnomalyRepository _anomalies;
+    private readonly IEventRepository _events;
+    private readonly ILogger<GetPendingAnomalyNotificationsUseCase> _logger;
+
+    public GetPendingAnomalyNotificationsUseCase(
+        IAnomalyRepository anomalies,
+        IEventRepository events,
+        ILogger<GetPendingAnomalyNotificationsUseCase> logger)
+    {
+        _anomalies = anomalies ?? throw new ArgumentNullException(nameof(anomalies));
+        _events = events ?? throw new ArgumentNullException(nameof(events));
+        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+    }
+
+    public async Task<IReadOnlyList<AnomalyNotification>> ExecuteAsync(
+        DateTimeOffset since,
+        decimal minScore,
+        CancellationToken ct)
+    {
+        // Date filter pushed to SQL; score filter is cheap in memory over the small slice.
+        var recent = await _anomalies.ListByDateRangeAsync(since, to: null, ct).ConfigureAwait(false);
+        var qualifying = recent.Where(a => a.Score >= minScore).ToList();
+        if (qualifying.Count == 0)
+            return Array.Empty<AnomalyNotification>();
+
+        var eventIds = qualifying.Select(a => a.EventId).Distinct().ToList();
+        var events = await _events.GetManyAsync(eventIds, ct).ConfigureAwait(false);
+
+        var notifications = qualifying
+            .OrderBy(a => a.DetectedAt)
+            .Select(a => new AnomalyNotification(
+                AnomalyId: a.Id,
+                EventTitle: events.TryGetValue(a.EventId, out var ev) ? ev.Title : a.EventId.Value,
+                Kind: a.Kind,
+                Score: a.Score,
+                DetectedAt: a.DetectedAt))
+            .ToList();
+
+        _logger.LogDebug(
+            "GetPendingAnomalyNotificationsUseCase: {Count} alert(s) since {Since:O} at minScore {MinScore}",
+            notifications.Count, since, minScore);
+
+        return notifications;
+    }
+}