feat: surface recommendation rationale

internal/anime/recommendations/rerank.go

@@ -24,7 +24,7 @@ func rerankRecommendationCandidates(candidates []recommendationCandidate, limit
 			continue
 		}
 
-		selected = append(selected, domain.Anime{Anime: candidate.anime})
+		selected = append(selected, domain.Anime{Anime: candidate.anime, RecommendationRationale: candidate.rationale})
 		features := diversityFeatures(candidate.anime)
 		seen.add(features)
 		recent = append(recent, features)

internal/anime/recommendations/scoring.go

@@ -48,9 +48,23 @@ func scoreRecommendationCandidate(
 		themeMatches:       themes,
 		studioMatches:      studios,
 		demographicMatches: demos,
+		rationale:          buildRecommendationRationale(profile, candidate),
 	}
 }
 
+func buildRecommendationRationale(profile userTasteProfile, candidate jikan.Anime) []string {
+	rationale := make([]string, 0, 4)
+	rationale = append(rationale, matchedEntityNames(profile.genres, candidate.Genres)...)
+	rationale = append(rationale, matchedEntityNames(profile.themes, candidate.Themes)...)
+	rationale = append(rationale, matchedEntityNames(profile.studios, candidate.Studios)...)
+	rationale = append(rationale, matchedEntityNames(profile.demographics, candidate.Demographics)...)
+
+	if len(rationale) > 4 {
+		return rationale[:4]
+	}
+	return rationale
+}
+
 func recommendationCandidateScoreAdjustments(now time.Time, profile userTasteProfile, candidate jikan.Anime) float64 {
 	var score float64
 
@@ -115,3 +129,22 @@ func weightedEntityMatch(weights map[int]float64, entities []jikan.NamedEntity)
 
 	return matches, score
 }
+
+func matchedEntityNames(weights map[int]float64, entities []jikan.NamedEntity) []string {
+	if len(weights) == 0 {
+		return []string{}
+	}
+
+	names := make([]string, 0, 1)
+	for _, entity := range entities {
+		if entity.Name == "" || weights[entity.MalID] <= 0 {
+			continue
+		}
+		names = append(names, entity.Name)
+		if len(names) >= 1 {
+			break
+		}
+	}
+
+	return names
+}

internal/anime/recommendations/types.go

@@ -25,6 +25,7 @@ type recommendationCandidate struct {
 	themeMatches       int
 	studioMatches      int
 	demographicMatches int
+	rationale          []string
 }
 
 type userTasteProfile struct {