feat(api): My Wave sessions with a Go fallback and wave metrics (M4)

- /wave/options, POST /wave/sessions (activity, mood, seed),
  /next with reactions, PATCH to switch the context on the fly, DELETE
- the recommender is called with an 800 ms budget; on failure the api
  serves a fallback wave of likes, favourite artists and random
  available tracks with the same dislike / recent / served filters
- every served track is an impression with its source and model; wave
  events (context_type wave) set its outcome, reactions win
- library status shows the analysis queue and 7-day wave metrics:
  listen-through, early skip and like rate

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

services/api/internal/playback/handler.go

@@ -268,11 +268,20 @@ func (h *Handler) PostEvents(ctx context.Context, req gen.PostEventsRequestObjec
 		if e.PlayedMs != nil {
 			played = *e.PlayedMs
 		}
-		if o := Judge(ev.Type, played, dur); o != (Outcome{}) {
+		o := Judge(ev.Type, played, dur)
+		if o != (Outcome{}) {
 			if err := h.store.ApplyOutcome(ctx, tx, p.UserID, ev, o); err != nil {
 				return nil, err
 			}
 		}
+		if outcome := impressionOutcome(ev.Type, o); outcome != "" && ev.ContextType != nil && *ev.ContextType == "wave" &&
+			ev.ContextID != nil {
+			if sid, err := uuid.Parse(*ev.ContextID); err == nil {
+				if err := h.store.MarkImpression(ctx, tx, sid, ev.TrackID, outcome); err != nil {
+					return nil, err
+				}
+			}
+		}
 	}
 	if err := tx.Commit(ctx); err != nil {
 		return nil, err
@@ -551,3 +560,18 @@ func (h *Handler) ClearTrackReaction(ctx context.Context, req gen.ClearTrackReac
 }
 
 var _ library.Personalizer = (*Handler)(nil)
+
+// impressionOutcome — исход показа волны по событию: complete | skip_early | skip | like | dislike.
+func impressionOutcome(eventType string, o Outcome) string {
+	switch {
+	case eventType == "like" || eventType == "dislike":
+		return eventType
+	case o.Complete:
+		return "complete"
+	case o.EarlySkip:
+		return "skip_early"
+	case o.Skip:
+		return "skip"
+	}
+	return ""
+}