feat(tuning): expose EnrichedTagScale in the tuning lab (#1520)

Promote the enriched-tag weight (#1490) from a taste.Config default into
the DB-backed tuning lab so operators can dial how much folksonomy tags
count vs raw ID3 genre (rule #25).

- Migration 0044: taste_tuning.enriched_tag_scale (DEFAULT 0.5, backfills
  the existing row).
- recsettings: TasteTuning gains the field; seeded/read/updated through
  reconcile + persistTaste; applyTastePatch validates it to [0,1]
  (generic non-half-life clamp) and diffTaste audits it; TasteConfig maps
  it into the profile build.
- API: tasteTuningResp exposes enriched_tag_scale.
- Web tuning card: a data-driven "Enriched tag weight" knob (0 = genre
  only). Tests: recsettings persist+range, web fixture field.

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

internal/recsettings/patch.go

@@ -129,6 +129,8 @@ func applyTastePatch(current TasteTuning, patch map[string]float64) (TasteTuning
 			target = &next.EngagementNeutral
 		case "engagement_full":
 			target = &next.EngagementFull
+		case "enriched_tag_scale":
+			target = &next.EnrichedTagScale
 		default:
 			return current, nil, fmt.Errorf("%w: %q", ErrUnknownField, field)
 		}
@@ -174,5 +176,6 @@ func diffTaste(a, b TasteTuning) []fieldChange {
 	add("engagement_hard_skip", a.EngagementHardSkip, b.EngagementHardSkip)
 	add("engagement_neutral", a.EngagementNeutral, b.EngagementNeutral)
 	add("engagement_full", a.EngagementFull, b.EngagementFull)
+	add("enriched_tag_scale", a.EnrichedTagScale, b.EnrichedTagScale)
 	return out
 }

internal/recsettings/service.go

@@ -38,12 +38,14 @@ const (
 )
 
 // TasteTuning is the tunable subset of taste.Config: the engagement
-// half-life and the completion→engagement curve points.
+// half-life, the completion→engagement curve points, and the enriched-tag
+// weight (how much folksonomy tags count vs raw ID3 genre, #1520).
 type TasteTuning struct {
 	HalfLifeDays       float64
 	EngagementHardSkip float64
 	EngagementNeutral  float64
 	EngagementFull     float64
+	EnrichedTagScale   float64
 }
 
 // ShippedRadioWeights are the shipped radio-profile defaults (moved
@@ -86,6 +88,7 @@ func ShippedTasteTuning() TasteTuning {
 		EngagementHardSkip: d.Engagement.HardSkip,
 		EngagementNeutral:  d.Engagement.NeutralCompletion,
 		EngagementFull:     d.Engagement.FullCompletion,
+		EnrichedTagScale:   d.EnrichedTagScale,
 	}
 }
 
@@ -133,6 +136,7 @@ func (s *Service) reconcile(ctx context.Context) error {
 		EngagementHardSkip: st.EngagementHardSkip,
 		EngagementNeutral:  st.EngagementNeutral,
 		EngagementFull:     st.EngagementFull,
+		EnrichedTagScale:   st.EnrichedTagScale,
 	}); err != nil {
 		return fmt.Errorf("seed taste tuning: %w", err)
 	}
@@ -156,6 +160,7 @@ func (s *Service) reconcile(ctx context.Context) error {
 		EngagementHardSkip: tt.EngagementHardSkip,
 		EngagementNeutral:  tt.EngagementNeutral,
 		EngagementFull:     tt.EngagementFull,
+		EnrichedTagScale:   tt.EnrichedTagScale,
 	}
 	s.mu.Unlock()
 
@@ -205,6 +210,7 @@ func (s *Service) TasteConfig() taste.Config {
 		NeutralCompletion: t.EngagementNeutral,
 		FullCompletion:    t.EngagementFull,
 	}
+	cfg.EnrichedTagScale = t.EnrichedTagScale
 	return cfg
 }
 
@@ -301,6 +307,7 @@ func (s *Service) persistTaste(
 		EngagementHardSkip: t.EngagementHardSkip,
 		EngagementNeutral:  t.EngagementNeutral,
 		EngagementFull:     t.EngagementFull,
+		EnrichedTagScale:   t.EnrichedTagScale,
 	}); err != nil {
 		return fmt.Errorf("update taste tuning: %w", err)
 	}

internal/recsettings/service_test.go

@@ -182,6 +182,27 @@ func TestUpdateTaste_CurveOrderingEnforced(t *testing.T) {
 	}
 }
 
+func TestUpdateTaste_EnrichedTagScale(t *testing.T) {
+	pool := newPool(t)
+	s := newService(t, pool)
+	// In-range update persists into cache + the assembled taste config.
+	if err := s.UpdateTaste(context.Background(),
+		map[string]float64{"enriched_tag_scale": 0.8}); err != nil {
+		t.Fatalf("UpdateTaste: %v", err)
+	}
+	if got := s.Taste().EnrichedTagScale; got != 0.8 {
+		t.Errorf("Taste().EnrichedTagScale = %v, want 0.8", got)
+	}
+	if got := s.TasteConfig().EnrichedTagScale; got != 0.8 {
+		t.Errorf("TasteConfig().EnrichedTagScale = %v, want 0.8", got)
+	}
+	// Out of [0,1] rejects.
+	if err := s.UpdateTaste(context.Background(),
+		map[string]float64{"enriched_tag_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)