feat(api): radio uses similarity pool with exclude param + M3 fallback

Wire LoadCandidatesFromSimilarity as the primary candidate loader with
an ?exclude= query param for comma-separated UUID filtering; fall back
to LoadCandidates on error. Thread SimilarityWeight into ScoringWeights
and update testHandlers recCfg accordingly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/api/radio.go

@@ -82,19 +82,33 @@ func (h *handlers) handleRadio(w http.ResponseWriter, r *http.Request) {
 
 	currentVec := loadCurrentSessionVector(r, q, user.ID, h.logger)
 
-	candidates, err := recommendation.LoadCandidates(r.Context(), q, user.ID, seedID, h.recCfg.RecentlyPlayedHours, currentVec)
+	exclude := parseExcludeParam(r.URL.Query().Get("exclude"))
+	limits := recommendation.DefaultCandidateSourceLimits()
+	candidates, err := recommendation.LoadCandidatesFromSimilarity(
+		r.Context(), q, user.ID, seedID,
+		h.recCfg.RecentlyPlayedHours, currentVec, exclude, limits,
+	)
 	if err != nil {
-		h.logger.Error("api: radio: load candidates", "err", err)
-		writeErr(w, http.StatusInternalServerError, "server_error", "candidate load failed")
-		return
+		h.logger.Warn("api: radio: similarity-pool failed; falling back to whole-library", "err", err)
+		candidates, err = recommendation.LoadCandidates(
+			r.Context(), q, user.ID, seedID,
+			h.recCfg.RecentlyPlayedHours, currentVec,
+		)
+		if err != nil {
+			h.logger.Error("api: radio: load candidates fallback failed", "err", err)
+			writeErr(w, http.StatusInternalServerError, "server_error", "candidate load failed")
+			return
+		}
 	}
+
 	weights := recommendation.ScoringWeights{
-		BaseWeight:      h.recCfg.BaseWeight,
-		LikeBoost:       h.recCfg.LikeBoost,
-		RecencyWeight:   h.recCfg.RecencyWeight,
-		SkipPenalty:     h.recCfg.SkipPenalty,
-		JitterMagnitude: h.recCfg.JitterMagnitude,
-		ContextWeight:   h.recCfg.ContextWeight,
+		BaseWeight:       h.recCfg.BaseWeight,
+		LikeBoost:        h.recCfg.LikeBoost,
+		RecencyWeight:    h.recCfg.RecencyWeight,
+		SkipPenalty:      h.recCfg.SkipPenalty,
+		JitterMagnitude:  h.recCfg.JitterMagnitude,
+		ContextWeight:    h.recCfg.ContextWeight,
+		SimilarityWeight: h.recCfg.SimilarityWeight,
 	}
 	picks := recommendation.Shuffle(candidates, weights, time.Now().UTC(), h.rng, limit-1)
 
@@ -141,3 +155,26 @@ func loadCurrentSessionVector(r *http.Request, q *dbq.Queries, userID pgtype.UUI
 	}
 	return v
 }
+
+// parseExcludeParam parses a comma-separated list of UUIDs from the
+// `exclude` query string, silently dropping malformed entries. Returns
+// nil for empty or all-malformed input.
+func parseExcludeParam(raw string) []pgtype.UUID {
+	if raw == "" {
+		return nil
+	}
+	parts := strings.Split(raw, ",")
+	out := make([]pgtype.UUID, 0, len(parts))
+	for _, p := range parts {
+		p = strings.TrimSpace(p)
+		if p == "" {
+			continue
+		}
+		id, ok := parseUUID(p)
+		if !ok {
+			continue
+		}
+		out = append(out, id)
+	}
+	return out
+}