fix(phase-7): close review notes — hoist anomaly dedup query, drop dead expr

Phase 7 reviewer (Sonnet, combined backend + frontend) flagged 3 🟡 warnings;
two real fixes here, one tracking:

W1 — DetectAnomaliesUseCase had an undocumented N+1: _anomalyRepo.ListAsync
  was called inside ProcessEventAsync, once per event. Hoisted to ExecuteAsync
  before the loop and threaded into ProcessEventAsync as a parameter. The
  per-event slice happens in-memory now. O(N_events) DB round-trips → 1.

W2 — AnomalyDetector.ExtractMatchWinProbabilities had a dead expression
  '(decimal?)null ?? 0m' that always evaluated to 0m. Simplified to
  'drawBet is not null ? rawDraw / total : 0m'. The 0m is never surfaced
  anyway (PDraw in the return uses the same null guard), so behaviour is
  identical.

W3 — PLAN.md row updated with both Phase 7 commit hashes (a6ff368 backend
  + 12208a4 frontend) and review verdict.

Build 0/0, 276 tests still passing.

src/Marathon.Application/UseCases/DetectAnomaliesUseCase.cs

@@ -70,13 +70,18 @@ public sealed class DetectAnomaliesUseCase
         var now  = DateTimeOffset.UtcNow.ToOffset(MoscowOffset);
         var from = now - SnapshotLookback;
 
+        // Hoisted outside the per-event loop: load existing anomalies ONCE per cycle
+        // and slice per-event in the loop. Previously this was reloaded per event
+        // (O(N_events) round-trips). Reviewer W1, Phase 7.
+        var existingAnomalies = await _anomalyRepo.ListAsync(ct);
+
         foreach (var ev in events)
         {
             ct.ThrowIfCancellationRequested();
 
             try
             {
-                newAnomalyCount += await ProcessEventAsync(detector, ev, from, now, ct);
+                newAnomalyCount += await ProcessEventAsync(detector, ev, from, now, existingAnomalies, ct);
             }
             catch (OperationCanceledException)
             {
@@ -104,6 +109,7 @@ public sealed class DetectAnomaliesUseCase
         Event ev,
         DateTimeOffset from,
         DateTimeOffset to,
+        IReadOnlyList<Anomaly> existingAnomalies,
         CancellationToken ct)
     {
         var snapshots = await _snapshotRepo.ListByEventAsync(ev.Id, from, to, ct);
@@ -112,9 +118,8 @@ public sealed class DetectAnomaliesUseCase
         if (detected.Count == 0)
             return 0;
 
-        // Load existing anomalies for this event so we can deduplicate.
-        var existing = await _anomalyRepo.ListAsync(ct);
-        var existingForEvent = existing
+        // Slice the cycle-wide existing-anomaly list to just this event for dedup.
+        var existingForEvent = existingAnomalies
             .Where(a => a.EventId == ev.Id)
             .ToList();