feat(taste): mood taste facet — #1534

Milestone #160 Opt 2b (mood half of the era+mood option). A fourth taste
facet alongside artists + genre tags + eras: signed weights over canonical
mood buckets (melancholic / energetic / chill / …) derived from a track's
enriched folksonomy tags (#1490).

- internal/mood: shared vocabulary — Of(tags) maps folksonomy tags to
  canonical mood buckets (synonyms collapse). Imported by both the taste
  builder and the scorer so a track's mood is derived identically.
- Migration 0047: taste_profile_moods table + taste_tuning.mood_scale
  (DEFAULT 0.5).
- Build side (internal/taste): Config.MoodScale ([0,1] damper, mirrors
  EraScale); accumulate folds each play/like's mood buckets at
  base*MoodScale; persist atomic-replaces the mood rows.
- Scorer (internal/recommendation): TasteProfile gains a mood term
  (own tanh scale + additive 0.12 share, so it never weakens the existing
  signal when a track has no mood tags). Match now takes the candidate's
  mood buckets; loaded per candidate (ListTrackTagsForTracks → mood.Of) in
  the primary similarity loader only — the near-whole-library fallback
  pool passes nil (mood → 0) to avoid a full-library tag scan.
- Tuning lab: mood_scale threaded through recsettings + admin API + web
  card ("Mood weight" row) + Go/web tests.

Coverage is partial (grows with tag enrichment; richer once Last.fm is
keyed), so mood is a supplement — neutral for tracks with no mood tags.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/recsettings/patch.go

@@ -137,6 +137,8 @@ func applyTastePatch(current TasteTuning, patch map[string]float64) (TasteTuning
 			target = &next.EnrichedTagScale
 		case "era_scale":
 			target = &next.EraScale
+		case "mood_scale":
+			target = &next.MoodScale
 		default:
 			return current, nil, fmt.Errorf("%w: %q", ErrUnknownField, field)
 		}
@@ -184,5 +186,6 @@ func diffTaste(a, b TasteTuning) []fieldChange {
 	add("engagement_full", a.EngagementFull, b.EngagementFull)
 	add("enriched_tag_scale", a.EnrichedTagScale, b.EnrichedTagScale)
 	add("era_scale", a.EraScale, b.EraScale)
+	add("mood_scale", a.MoodScale, b.MoodScale)
 	return out
 }

internal/recsettings/service.go

@@ -48,6 +48,7 @@ type TasteTuning struct {
 	EngagementFull     float64
 	EnrichedTagScale   float64
 	EraScale           float64
+	MoodScale          float64
 }
 
 // ShippedRadioWeights are the shipped radio-profile defaults (moved
@@ -96,6 +97,7 @@ func ShippedTasteTuning() TasteTuning {
 		EngagementFull:     d.Engagement.FullCompletion,
 		EnrichedTagScale:   d.EnrichedTagScale,
 		EraScale:           d.EraScale,
+		MoodScale:          d.MoodScale,
 	}
 }
 
@@ -145,6 +147,7 @@ func (s *Service) reconcile(ctx context.Context) error {
 		EngagementFull:     st.EngagementFull,
 		EnrichedTagScale:   st.EnrichedTagScale,
 		EraScale:           st.EraScale,
+		MoodScale:          st.MoodScale,
 	}); err != nil {
 		return fmt.Errorf("seed taste tuning: %w", err)
 	}
@@ -170,6 +173,7 @@ func (s *Service) reconcile(ctx context.Context) error {
 		EngagementFull:     tt.EngagementFull,
 		EnrichedTagScale:   tt.EnrichedTagScale,
 		EraScale:           tt.EraScale,
+		MoodScale:          tt.MoodScale,
 	}
 	s.mu.Unlock()
 
@@ -221,6 +225,7 @@ func (s *Service) TasteConfig() taste.Config {
 	}
 	cfg.EnrichedTagScale = t.EnrichedTagScale
 	cfg.EraScale = t.EraScale
+	cfg.MoodScale = t.MoodScale
 	return cfg
 }
 
@@ -319,6 +324,7 @@ func (s *Service) persistTaste(
 		EngagementFull:     t.EngagementFull,
 		EnrichedTagScale:   t.EnrichedTagScale,
 		EraScale:           t.EraScale,
+		MoodScale:          t.MoodScale,
 	}); err != nil {
 		return fmt.Errorf("update taste tuning: %w", err)
 	}

internal/recsettings/service_test.go

@@ -224,6 +224,25 @@ func TestUpdateTaste_EraScale(t *testing.T) {
 	}
 }
 
+func TestUpdateTaste_MoodScale(t *testing.T) {
+	pool := newPool(t)
+	s := newService(t, pool)
+	if err := s.UpdateTaste(context.Background(),
+		map[string]float64{"mood_scale": 0.8}); err != nil {
+		t.Fatalf("UpdateTaste: %v", err)
+	}
+	if got := s.Taste().MoodScale; got != 0.8 {
+		t.Errorf("Taste().MoodScale = %v, want 0.8", got)
+	}
+	if got := s.TasteConfig().MoodScale; got != 0.8 {
+		t.Errorf("TasteConfig().MoodScale = %v, want 0.8", got)
+	}
+	if err := s.UpdateTaste(context.Background(),
+		map[string]float64{"mood_scale": 1.5}); !errors.Is(err, ErrOutOfRange) {
+		t.Errorf("err = %v, want ErrOutOfRange for 1.5", err)
+	}
+}
+
 func TestReset_RestoresShippedAndAudits(t *testing.T) {
 	pool := newPool(t)
 	s := newService(t, pool)