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.go

@@ -38,14 +38,16 @@ const (
 )
 
 // TasteTuning is the tunable subset of taste.Config: the engagement
-// half-life, the completion→engagement curve points, and the enriched-tag
-// weight (how much folksonomy tags count vs raw ID3 genre, #1520).
+// half-life, the completion→engagement curve points, the enriched-tag
+// weight (how much folksonomy tags count vs raw ID3 genre, #1520), and the
+// era-facet weight (how strongly a decade-play imprints, #1530).
 type TasteTuning struct {
 	HalfLifeDays       float64
 	EngagementHardSkip float64
 	EngagementNeutral  float64
 	EngagementFull     float64
 	EnrichedTagScale   float64
+	EraScale           float64
 }
 
 // ShippedRadioWeights are the shipped radio-profile defaults (moved
@@ -89,6 +91,7 @@ func ShippedTasteTuning() TasteTuning {
 		EngagementNeutral:  d.Engagement.NeutralCompletion,
 		EngagementFull:     d.Engagement.FullCompletion,
 		EnrichedTagScale:   d.EnrichedTagScale,
+		EraScale:           d.EraScale,
 	}
 }
 
@@ -137,6 +140,7 @@ func (s *Service) reconcile(ctx context.Context) error {
 		EngagementNeutral:  st.EngagementNeutral,
 		EngagementFull:     st.EngagementFull,
 		EnrichedTagScale:   st.EnrichedTagScale,
+		EraScale:           st.EraScale,
 	}); err != nil {
 		return fmt.Errorf("seed taste tuning: %w", err)
 	}
@@ -161,6 +165,7 @@ func (s *Service) reconcile(ctx context.Context) error {
 		EngagementNeutral:  tt.EngagementNeutral,
 		EngagementFull:     tt.EngagementFull,
 		EnrichedTagScale:   tt.EnrichedTagScale,
+		EraScale:           tt.EraScale,
 	}
 	s.mu.Unlock()
 
@@ -211,6 +216,7 @@ func (s *Service) TasteConfig() taste.Config {
 		FullCompletion:    t.EngagementFull,
 	}
 	cfg.EnrichedTagScale = t.EnrichedTagScale
+	cfg.EraScale = t.EraScale
 	return cfg
 }
 
@@ -308,6 +314,7 @@ func (s *Service) persistTaste(
 		EngagementNeutral:  t.EngagementNeutral,
 		EngagementFull:     t.EngagementFull,
 		EnrichedTagScale:   t.EnrichedTagScale,
+		EraScale:           t.EraScale,
 	}); err != nil {
 		return fmt.Errorf("update taste tuning: %w", err)
 	}