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/db/dbq/models.go

@@ -568,6 +568,7 @@ type TasteTuning struct {
 	EngagementNeutral  float64
 	EngagementFull     float64
 	UpdatedAt          pgtype.Timestamptz
+	EnrichedTagScale   float64
 }
 
 type Track struct {