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/dbq/recommendation.sql.go

@@ -585,8 +585,10 @@ SELECT
     (l.user_id IS NOT NULL)::bool AS is_liked,
     pe.last_played_at::timestamptz AS last_played_at,
     pe.play_count,
-    pe.skip_count
+    pe.skip_count,
+    al.release_date AS release_date
 FROM tracks t
+JOIN albums al ON al.id = t.album_id
 LEFT JOIN general_likes l ON l.user_id = $1 AND l.track_id = t.id
 LEFT JOIN LATERAL (
     SELECT
@@ -620,6 +622,7 @@ type LoadRadioCandidatesRow struct {
 	LastPlayedAt pgtype.Timestamptz
 	PlayCount    int64
 	SkipCount    int64
+	ReleaseDate  pgtype.Date
 }
 
 // Returns all tracks except the seed and any played by the user within
@@ -660,6 +663,7 @@ func (q *Queries) LoadRadioCandidates(ctx context.Context, arg LoadRadioCandidat
 			&i.LastPlayedAt,
 			&i.PlayCount,
 			&i.SkipCount,
+			&i.ReleaseDate,
 		); err != nil {
 			return nil, err
 		}
@@ -775,6 +779,7 @@ SELECT
     pe.last_played_at::timestamptz AS last_played_at,
     pe.play_count,
     pe.skip_count,
+    al.release_date AS release_date,
     COALESCE(max(u.sim_score), 0.0) AS similarity_score
 FROM (
     SELECT track_id, sim_score FROM lb_similar
@@ -785,6 +790,7 @@ FROM (
     UNION ALL SELECT track_id, sim_score FROM random_fill
 ) u
 JOIN tracks t ON t.id = u.track_id
+JOIN albums al ON al.id = t.album_id
 LEFT JOIN general_likes l ON l.user_id = $1 AND l.track_id = t.id
 LEFT JOIN LATERAL (
     SELECT max(started_at) AS last_played_at,
@@ -796,7 +802,8 @@ LEFT JOIN LATERAL (
 GROUP BY t.id, t.title, t.album_id, t.artist_id, t.duration_ms, t.file_path,
          t.file_format, t.file_size, t.bitrate, t.track_number, t.disc_number,
          t.mbid, t.genre, t.added_at, t.updated_at,
-         l.user_id, pe.last_played_at, pe.play_count, pe.skip_count
+         l.user_id, pe.last_played_at, pe.play_count, pe.skip_count,
+         al.release_date
 `
 
 type LoadRadioCandidatesV2Params struct {
@@ -818,6 +825,7 @@ type LoadRadioCandidatesV2Row struct {
 	LastPlayedAt    pgtype.Timestamptz
 	PlayCount       int64
 	SkipCount       int64
+	ReleaseDate     pgtype.Date
 	SimilarityScore interface{}
 }
 
@@ -874,6 +882,7 @@ func (q *Queries) LoadRadioCandidatesV2(ctx context.Context, arg LoadRadioCandid
 			&i.LastPlayedAt,
 			&i.PlayCount,
 			&i.SkipCount,
+			&i.ReleaseDate,
 			&i.SimilarityScore,
 		); err != nil {
 			return nil, err