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/api/admin_recommendation_tuning.go

@@ -49,6 +49,7 @@ type tasteTuningResp struct {
 	EngagementNeutral  float64 `json:"engagement_neutral"`
 	EngagementFull     float64 `json:"engagement_full"`
 	EnrichedTagScale   float64 `json:"enriched_tag_scale"`
+	EraScale           float64 `json:"era_scale"`
 }
 
 func tasteRespFrom(t recsettings.TasteTuning) tasteTuningResp {
@@ -58,6 +59,7 @@ func tasteRespFrom(t recsettings.TasteTuning) tasteTuningResp {
 		EngagementNeutral:  t.EngagementNeutral,
 		EngagementFull:     t.EngagementFull,
 		EnrichedTagScale:   t.EnrichedTagScale,
+		EraScale:           t.EraScale,
 	}
 }