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/api/admin_recommendation_tuning.go

@@ -48,6 +48,7 @@ type tasteTuningResp struct {
 	EngagementHardSkip float64 `json:"engagement_hard_skip"`
 	EngagementNeutral  float64 `json:"engagement_neutral"`
 	EngagementFull     float64 `json:"engagement_full"`
+	EnrichedTagScale   float64 `json:"enriched_tag_scale"`
 }
 
 func tasteRespFrom(t recsettings.TasteTuning) tasteTuningResp {
@@ -56,6 +57,7 @@ func tasteRespFrom(t recsettings.TasteTuning) tasteTuningResp {
 		EngagementHardSkip: t.EngagementHardSkip,
 		EngagementNeutral:  t.EngagementNeutral,
 		EngagementFull:     t.EngagementFull,
+		EnrichedTagScale:   t.EnrichedTagScale,
 	}
 }