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/patch.go

@@ -131,6 +131,8 @@ func applyTastePatch(current TasteTuning, patch map[string]float64) (TasteTuning
 			target = &next.EngagementFull
 		case "enriched_tag_scale":
 			target = &next.EnrichedTagScale
+		case "era_scale":
+			target = &next.EraScale
 		default:
 			return current, nil, fmt.Errorf("%w: %q", ErrUnknownField, field)
 		}
@@ -177,5 +179,6 @@ func diffTaste(a, b TasteTuning) []fieldChange {
 	add("engagement_neutral", a.EngagementNeutral, b.EngagementNeutral)
 	add("engagement_full", a.EngagementFull, b.EngagementFull)
 	add("enriched_tag_scale", a.EnrichedTagScale, b.EnrichedTagScale)
+	add("era_scale", a.EraScale, b.EraScale)
 	return out
 }