feat(admin): wave metrics split by model version (M6)

The library page compares listen-through, early-skip and like rates per
impression model version, so v1 and the trained ranker can be compared online.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

services/api/internal/library/handler.go

@@ -308,9 +308,15 @@ func (h *Handler) GetLibraryStatus(ctx context.Context, _ gen.GetLibraryStatusRe
 		},
 		Wave: gen.WaveMetrics{
 			Served: st.Wave.Served, Sessions: st.Wave.Sessions, ListenThroughRate: st.Wave.ListenThrough,
-			EarlySkipRate: st.Wave.EarlySkip, LikeRate: st.Wave.LikeRate,
+			EarlySkipRate: st.Wave.EarlySkip, LikeRate: st.Wave.LikeRate, ByModel: []gen.WaveModelMetrics{},
 		},
 	}
+	for _, m := range st.Wave.ByModel {
+		out.Wave.ByModel = append(out.Wave.ByModel, gen.WaveModelMetrics{
+			ModelVer: m.ModelVer, Served: m.Served, ListenThroughRate: m.ListenThrough,
+			EarlySkipRate: m.EarlySkip, LikeRate: m.LikeRate,
+		})
+	}
 	for _, s := range st.Sources {
 		dto := h.sourceDTO(s)
 		out.Sources = append(out.Sources, gen.SourceStatusItem{