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/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)