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/migrations/0044_taste_tuning_enriched_tag_scale.down.sql

@@ -0,0 +1,2 @@
+-- Reverse 0044_taste_tuning_enriched_tag_scale.up.sql.
+ALTER TABLE taste_tuning DROP COLUMN IF EXISTS enriched_tag_scale;

internal/db/migrations/0044_taste_tuning_enriched_tag_scale.up.sql

@@ -0,0 +1,6 @@
+-- Expose EnrichedTagScale in the tuning lab (#1520). Adds the enriched-tag
+-- weight knob to the taste_tuning singleton so operators can dial how much
+-- folksonomy tags (#1490) count vs raw ID3 genre. DEFAULT 0.5 backfills the
+-- existing row and matches taste.DefaultConfig().EnrichedTagScale.
+ALTER TABLE taste_tuning
+    ADD COLUMN enriched_tag_scale double precision NOT NULL DEFAULT 0.5;