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/queries/recommendation_tuning.sql

@@ -30,8 +30,8 @@ RETURNING *;
 -- name: UpsertTasteTuningDefaults :exec
 INSERT INTO taste_tuning (
     singleton, half_life_days, engagement_hard_skip,
-    engagement_neutral, engagement_full
-) VALUES (true, $1, $2, $3, $4)
+    engagement_neutral, engagement_full, enriched_tag_scale
+) VALUES (true, $1, $2, $3, $4, $5)
 ON CONFLICT (singleton) DO NOTHING;
 
 -- name: GetTasteTuning :one
@@ -43,6 +43,7 @@ UPDATE taste_tuning
        engagement_hard_skip = $2,
        engagement_neutral   = $3,
        engagement_full      = $4,
+       enriched_tag_scale   = $5,
        updated_at           = now()
  WHERE singleton = true
 RETURNING *;