feat(anomaly): overround-compression detector

Adds the 4th IAnomalyDetector: flags a sharp drop in the bookmaker's
overround (raw implied-probability sum / margin) over a continuous live
window. Informational/non-directional — excluded from outcome grading
and backtest staking via AnomalyKind.IsDirectional().

- OverroundCompressionDetector: sliding-window scan mirroring SteamMove,
  score = min(1, compression / 0.10 reference), suspension-gap aware.
- MatchWinEvidence.Probabilities gains Overround (pre-normalisation
  margin); evidence JSON shape unchanged.
- Wired into DetectAnomaliesUseCase fan-out; AnomalyOptions + appsettings
  (window 120s, threshold 0.02); en/ru resx + KindLabel arms.
- 11 detector tests incl. score saturation/scaling, inclusive threshold
  boundary, and a three-way (draw-leg) overround case.

src/Marathon.Application/UseCases/DetectAnomaliesUseCase.cs

@@ -74,6 +74,11 @@ public sealed class DetectAnomaliesUseCase
                 _options.SuspensionGapSeconds,
                 _options.SuspensionFreezeThreshold,
                 _options.MinSnapshotCount),
+            new OverroundCompressionDetector(
+                _options.OverroundWindowSeconds,
+                _options.OverroundCompressionThreshold,
+                _options.MinSnapshotCount,
+                _options.SuspensionGapSeconds),
         };
 
         var events = await _eventRepo.ListAsync(ct);