feat(taste): era/decade taste facet — #1530

Milestone #160 Opt 2 (era half). A third taste facet alongside artists
+ genre tags: signed weights over decade buckets ("1990s") derived from
albums.release_date, rebuilt daily and scored into the taste match.

- Migration 0045: taste_profile_eras table (mirrors taste_profile_tags)
  + taste_tuning.era_scale column (DEFAULT 0.5).
- Build side (internal/taste): Config.EraScale ([0,1] damper, mirrors
  EnrichedTagScale), accumulate folds each play/like's decade at
  base*EraScale, persist atomic-replaces the era rows.
- Scorer (internal/recommendation): TasteProfile gains an era term (own
  tanh scale + additive 0.15 share so it never weakens the existing
  artist/tag signal when a track is undated); candidate queries return
  album release_date; decadeOf mirrors the builder helper.
- Tuning lab: era_scale threaded through recsettings + admin API + web
  card (auto-renders the new row) + Go/web tests.

Mood facet deferred to #1534 (partial enrichment coverage + needs
candidate-side enriched-tag loading).

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, enriched_tag_scale
-) VALUES (true, $1, $2, $3, $4, $5)
+    engagement_neutral, engagement_full, enriched_tag_scale, era_scale
+) VALUES (true, $1, $2, $3, $4, $5, $6)
 ON CONFLICT (singleton) DO NOTHING;
 
 -- name: GetTasteTuning :one
@@ -44,6 +44,7 @@ UPDATE taste_tuning
        engagement_neutral   = $3,
        engagement_full      = $4,
        enriched_tag_scale   = $5,
+       era_scale            = $6,
        updated_at           = now()
  WHERE singleton = true
 RETURNING *;