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/recsettings/patch.go

@@ -129,6 +129,8 @@ func applyTastePatch(current TasteTuning, patch map[string]float64) (TasteTuning
 			target = &next.EngagementNeutral
 		case "engagement_full":
 			target = &next.EngagementFull
+		case "enriched_tag_scale":
+			target = &next.EnrichedTagScale
 		default:
 			return current, nil, fmt.Errorf("%w: %q", ErrUnknownField, field)
 		}
@@ -174,5 +176,6 @@ func diffTaste(a, b TasteTuning) []fieldChange {
 	add("engagement_hard_skip", a.EngagementHardSkip, b.EngagementHardSkip)
 	add("engagement_neutral", a.EngagementNeutral, b.EngagementNeutral)
 	add("engagement_full", a.EngagementFull, b.EngagementFull)
+	add("enriched_tag_scale", a.EnrichedTagScale, b.EnrichedTagScale)
 	return out
 }