feat(taste): era/decade taste facet — #1530

Milestone #160 Opt 2 (era half). A third taste facet alongside artists
+ genre tags: signed weights over decade buckets ("1990s") derived from
albums.release_date, rebuilt daily and scored into the taste match.

- Migration 0045: taste_profile_eras table (mirrors taste_profile_tags)
  + taste_tuning.era_scale column (DEFAULT 0.5).
- Build side (internal/taste): Config.EraScale ([0,1] damper, mirrors
  EnrichedTagScale), accumulate folds each play/like's decade at
  base*EraScale, persist atomic-replaces the era rows.
- Scorer (internal/recommendation): TasteProfile gains an era term (own
  tanh scale + additive 0.15 share so it never weakens the existing
  artist/tag signal when a track is undated); candidate queries return
  album release_date; decadeOf mirrors the builder helper.
- Tuning lab: era_scale threaded through recsettings + admin API + web
  card (auto-renders the new row) + Go/web tests.

Mood facet deferred to #1534 (partial enrichment coverage + needs
candidate-side enriched-tag loading).

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

internal/recsettings/service_test.go

@@ -203,6 +203,27 @@ func TestUpdateTaste_EnrichedTagScale(t *testing.T) {
 	}
 }
 
+func TestUpdateTaste_EraScale(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{"era_scale": 0.8}); err != nil {
+		t.Fatalf("UpdateTaste: %v", err)
+	}
+	if got := s.Taste().EraScale; got != 0.8 {
+		t.Errorf("Taste().EraScale = %v, want 0.8", got)
+	}
+	if got := s.TasteConfig().EraScale; got != 0.8 {
+		t.Errorf("TasteConfig().EraScale = %v, want 0.8", got)
+	}
+	// Out of [0,1] rejects.
+	if err := s.UpdateTaste(context.Background(),
+		map[string]float64{"era_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)