test: add recommendation rationale coverage

internal/anime/recommendations/recommendations_test.go

@@ -74,6 +74,47 @@ func TestScoreRecommendationCandidateRewardsProfileOverlap(t *testing.T) {
 	}
 }
 
+func TestScoreRecommendationCandidateBuildsRationaleFromProfileMatches(t *testing.T) {
+	now := time.Date(2026, time.June, 4, 12, 0, 0, 0, time.UTC)
+	profile := userTasteProfile{
+		genres:       map[int]float64{1: 2.0},
+		themes:       map[int]float64{10: 1.5},
+		studios:      map[int]float64{20: 1.0},
+		demographics: map[int]float64{30: 1.0},
+	}
+
+	candidate := scoreRecommendationCandidate(now, profile, jikan.Anime{
+		MalID:        10,
+		Genres:       []jikan.NamedEntity{{MalID: 1, Name: "Action"}},
+		Themes:       []jikan.NamedEntity{{MalID: 10, Name: "School"}},
+		Studios:      []jikan.NamedEntity{{MalID: 20, Name: "Production I.G"}},
+		Demographics: []jikan.NamedEntity{{MalID: 30, Name: "Shounen"}},
+	}, 5.0, 0)
+
+	want := []string{"Action", "School", "Production I.G", "Shounen"}
+	if !slices.Equal(candidate.rationale, want) {
+		t.Fatalf("expected profile match rationale %v, got %v", want, candidate.rationale)
+	}
+}
+
+func TestScoreRecommendationCandidateOmitsRationaleWhenSignalsAreWeak(t *testing.T) {
+	now := time.Date(2026, time.June, 4, 12, 0, 0, 0, time.UTC)
+	profile := userTasteProfile{
+		genres:       map[int]float64{1: 2.0},
+		themes:       map[int]float64{},
+		studios:      map[int]float64{},
+		demographics: map[int]float64{},
+	}
+
+	candidate := scoreRecommendationCandidate(now, profile, jikan.Anime{
+		MalID: 10,
+	}, 5.0, 0)
+
+	if len(candidate.rationale) != 0 {
+		t.Fatalf("expected no rationale for weak signals, got %v", candidate.rationale)
+	}
+}
+
 func TestBuildTasteProfileUsesSeedWeights(t *testing.T) {
 	now := time.Date(2026, time.June, 4, 12, 0, 0, 0, time.UTC)
 

templates/renderer_test.go

@@ -107,6 +107,37 @@ func TestRenderWithNonStringFragment(t *testing.T) {
 	}
 }
 
+func TestTopPicksTemplateRendersRecommendationRationale(t *testing.T) {
+	r, err := ProvideRenderer()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var buf bytes.Buffer
+	err = r.ExecuteFragment(&buf, "top_picks.gohtml", "content", map[string]any{
+		"Animes": []domain.Anime{
+			{
+				Anime: jikan.Anime{
+					MalID: 1,
+					Title: "Haikyuu!!",
+				},
+				RecommendationRationale: []string{"Sports", "Production I.G"},
+			},
+		},
+		"WatchlistMap": map[int64]bool{},
+	})
+	if err != nil {
+		t.Fatalf("ExecuteFragment error: %v", err)
+	}
+
+	body := buf.String()
+	for _, want := range []string{"Why this was picked", "Sports", "Production I.G"} {
+		if !strings.Contains(body, want) {
+			t.Fatalf("top picks template missing %q in output:\n%s", want, body)
+		}
+	}
+}
+
 func TestWatchTemplateEscapesJSONDataAttributes(t *testing.T) {
 	r, err := ProvideRenderer()
 	if err != nil {