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.sql

@@ -10,8 +10,10 @@ 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
@@ -145,6 +147,7 @@ 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
@@ -155,6 +158,7 @@ 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,
@@ -166,7 +170,8 @@ 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;
 
 -- name: SuggestArtistsForUser :many
 -- M5c: per-user artist suggestions ranked by signal x similarity. The