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/taste_profile.sql

@@ -6,11 +6,13 @@
 -- One row per play in the decay-relevant window. completion is the play's
 -- completion ratio (precomputed column when present, else duration_played /
 -- track duration, clamped to [0,1]); age_days drives the time-decay. Genre
--- is split into tags in Go. Quarantined tracks are excluded.
+-- is split into tags in Go; release_date derives the decade for the era
+-- facet (#1530). Quarantined tracks are excluded.
 SELECT
     t.id AS track_id,
     t.artist_id,
     t.genre,
+    a.release_date,
     LEAST(GREATEST(
         COALESCE(pe.completion_ratio,
                  pe.duration_played_ms::float8 / NULLIF(t.duration_ms, 0),
@@ -18,6 +20,7 @@ SELECT
     (EXTRACT(epoch FROM now() - pe.started_at) / 86400.0)::float8 AS age_days
 FROM play_events pe
 JOIN tracks t ON t.id = pe.track_id
+JOIN albums a ON a.id = t.album_id
 WHERE pe.user_id = $1
   AND pe.started_at > now() - ($2::float8 * INTERVAL '1 day')
   AND NOT EXISTS (
@@ -26,12 +29,13 @@ WHERE pe.user_id = $1
   );
 
 -- name: ListLikedTrackTasteInputsForUser :many
--- (track_id, artist_id, genre) for each track the user has explicitly
--- liked. Feeds the track-like bonus into the liked track's artist and
--- tags; track_id keys the enriched track_tags lookup (#1490).
-SELECT t.id AS track_id, t.artist_id, t.genre
+-- (track_id, artist_id, genre, release_date) for each track the user has
+-- explicitly liked. Feeds the track-like bonus into the liked track's artist,
+-- tags, and era (#1530); track_id keys the enriched track_tags lookup (#1490).
+SELECT t.id AS track_id, t.artist_id, t.genre, a.release_date
 FROM general_likes gl
 JOIN tracks t ON t.id = gl.track_id
+JOIN albums a ON a.id = t.album_id
 WHERE gl.user_id = $1;
 
 -- name: ListLikedArtistIDsForUser :many
@@ -66,3 +70,18 @@ FROM taste_profile_tags
 WHERE user_id = $1
 ORDER BY weight DESC
 LIMIT $2;
+
+-- name: DeleteTasteProfileErasForUser :exec
+DELETE FROM taste_profile_eras WHERE user_id = $1;
+
+-- name: InsertTasteProfileEra :exec
+INSERT INTO taste_profile_eras (user_id, era, weight)
+VALUES ($1, $2, $3);
+
+-- name: ListTasteProfileErasForUser :many
+-- Top-weighted taste eras (#1530); consumed by the scorer's era term.
+SELECT era, weight
+FROM taste_profile_eras
+WHERE user_id = $1
+ORDER BY weight DESC
+LIMIT $2;