feat(recommendation): extend Score with SimilarityScore + SimilarityWeight

internal/recommendation/score.go

@@ -11,23 +11,26 @@ import (
 // ContextualMatchScore is in [0, 1] — max similarity between the user's
 // current session vector and any non-seed contextual_like row for this
 // track. Set by LoadCandidates after a bulk fetch.
+// SimilarityScore is in [0, 1]; 0 when no signal (random fill).
 type ScoringInputs struct {
 	IsGeneralLiked       bool
 	LastPlayedAt         *time.Time // nil = never played
 	PlayCount            int        // total play_events
 	SkipCount            int        // play_events with was_skipped=true
 	ContextualMatchScore float64    // [0, 1]; 0 when no signal
+	SimilarityScore      float64    // [0, 1]; 0 when no signal (random fill)
 }
 
 // ScoringWeights are the operator-tunable knobs. Defaults live in
 // config.RecommendationConfig and are propagated here per request.
 type ScoringWeights struct {
-	BaseWeight      float64
-	LikeBoost       float64
-	RecencyWeight   float64
-	SkipPenalty     float64
-	JitterMagnitude float64
-	ContextWeight   float64
+	BaseWeight       float64
+	LikeBoost        float64
+	RecencyWeight    float64
+	SkipPenalty      float64
+	JitterMagnitude  float64
+	ContextWeight    float64
+	SimilarityWeight float64
 }
 
 // Score computes the weighted-shuffle score per spec §6:
@@ -37,6 +40,7 @@ type ScoringWeights struct {
 //	      + recency_decay * RecencyWeight
 //	      - skip_ratio * SkipPenalty
 //	      + contextual_match_score * ContextWeight
+//	      + similarity_score * SimilarityWeight
 //	      + small_random_jitter
 //
 // Higher score = more likely to surface. rng is a function returning a
@@ -50,6 +54,7 @@ func Score(in ScoringInputs, w ScoringWeights, now time.Time, rng func() float64
 	s += recencyDecay(in.LastPlayedAt, now) * w.RecencyWeight
 	s -= skipRatio(in.PlayCount, in.SkipCount) * w.SkipPenalty
 	s += in.ContextualMatchScore * w.ContextWeight
+	s += in.SimilarityScore * w.SimilarityWeight
 	s += (rng()*2 - 1) * w.JitterMagnitude
 	return s
 }