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/taste/profile.go

@@ -46,6 +46,14 @@ type Config struct {
 	// contribution entirely (falls back to genre-only).
 	EnrichedTagScale float64
 
+	// EraScale weights the decade/era facet (#1530): each play adds
+	// base × this scale to its album's decade bucket ("1990s"), where base
+	// is the decayed engagement (or the tag-like bonus for a liked track).
+	// Era is a coarse signal, so it's kept ≤ 1 to imprint at most as strongly
+	// as the raw engagement; 0 disables the era facet entirely. Tunable in the
+	// lab (mirrors EnrichedTagScale, #1520).
+	EraScale float64
+
 	// ArtistFloor / TagFloor clamp how negative a single entity's weight may
 	// go. Aggregation already protects an artist the user likes (one skip
 	// nets out against many good plays); the floor additionally bounds the
@@ -58,11 +66,15 @@ type Config struct {
 	WeightEpsilon float64
 
 	// Size caps (guard rails for pathological libraries). Persist the top
-	// MaxArtists by weight plus the most-negative MaxNegArtists; same for tags.
+	// MaxArtists by weight plus the most-negative MaxNegArtists; same for tags
+	// and eras. Real libraries span ~a dozen decades, so the era caps rarely
+	// bind — they exist for symmetry with the other facets.
 	MaxArtists    int
 	MaxNegArtists int
 	MaxTags       int
 	MaxNegTags    int
+	MaxEras       int
+	MaxNegEras    int
 }
 
 // DefaultConfig returns the tuned starting configuration.
@@ -75,6 +87,7 @@ func DefaultConfig() Config {
 		TrackLikeBonus:   1.0,
 		TagLikeBonus:     0.5,
 		EnrichedTagScale: 0.5,
+		EraScale:         0.5,
 		ArtistFloor:      -3.0,
 		TagFloor:         -3.0,
 		WeightEpsilon:    0.05,
@@ -82,6 +95,8 @@ func DefaultConfig() Config {
 		MaxNegArtists:    150,
 		MaxTags:          200,
 		MaxNegTags:       60,
+		MaxEras:          30,
+		MaxNegEras:       15,
 	}
 }
 
@@ -101,36 +116,40 @@ func BuildTasteProfile(
 ) error {
 	q := dbq.New(pool)
 
-	artistW, tagW, err := accumulate(ctx, q, userID, cfg)
+	artistW, tagW, eraW, err := accumulate(ctx, q, userID, cfg)
 	if err != nil {
 		return err
 	}
 	applyFloor(artistW, cfg.ArtistFloor)
 	applyFloor(tagW, cfg.TagFloor)
+	// Era shares the tag floor (both are text-keyed facets; a decade that's
+	// been heavily skipped shouldn't dominate as an extreme negative either).
+	applyFloor(eraW, cfg.TagFloor)
 
+	strLess := func(a, b string) bool { return a < b }
 	artists := rankWeighted(artistW, cfg.WeightEpsilon, cfg.MaxArtists, cfg.MaxNegArtists, uuidLess)
-	tags := rankWeighted(tagW, cfg.WeightEpsilon, cfg.MaxTags, cfg.MaxNegTags,
-		func(a, b string) bool { return a < b })
+	tags := rankWeighted(tagW, cfg.WeightEpsilon, cfg.MaxTags, cfg.MaxNegTags, strLess)
+	eras := rankWeighted(eraW, cfg.WeightEpsilon, cfg.MaxEras, cfg.MaxNegEras, strLess)
 
-	if err := persist(ctx, pool, userID, artists, tags); err != nil {
+	if err := persist(ctx, pool, userID, artists, tags, eras); err != nil {
 		return err
 	}
-	logger.Debug("taste: profile rebuilt",
-		"user_id", uuidString(userID), "artists", len(artists), "tags", len(tags))
+	logger.Debug("taste: profile rebuilt", "user_id", uuidString(userID),
+		"artists", len(artists), "tags", len(tags), "eras", len(eras))
 	return nil
 }
 
-// accumulate sums decayed play engagement and like bonuses into artist/tag
+// accumulate sums decayed play engagement and like bonuses into artist/tag/era
 // weight maps.
 func accumulate(
 	ctx context.Context, q *dbq.Queries, userID pgtype.UUID, cfg Config,
-) (map[pgtype.UUID]float64, map[string]float64, error) {
+) (map[pgtype.UUID]float64, map[string]float64, map[string]float64, error) {
 	plays, err := q.ListPlayEngagementInputsForUser(ctx, dbq.ListPlayEngagementInputsForUserParams{
 		UserID:  userID,
 		Column2: cfg.WindowDays,
 	})
 	if err != nil {
-		return nil, nil, fmt.Errorf("taste: load play engagement: %w", err)
+		return nil, nil, nil, fmt.Errorf("taste: load play engagement: %w", err)
 	}
 	// Enriched folksonomy tags keyed by track (#1490) — folded into the tag
 	// facet alongside raw ID3 genre so a coarse "Rock" gains the cached
@@ -141,12 +160,13 @@ func accumulate(
 		Column2: int32(cfg.WindowDays),
 	})
 	if err != nil {
-		return nil, nil, fmt.Errorf("taste: load played track tags: %w", err)
+		return nil, nil, nil, fmt.Errorf("taste: load played track tags: %w", err)
 	}
 	playedTags := groupTagsByTrack(playedTagRows)
 
 	artistW := make(map[pgtype.UUID]float64)
 	tagW := make(map[string]float64)
+	eraW := make(map[string]float64)
 	for _, p := range plays {
 		e := Engagement(p.Completion, cfg.Engagement) * decay(p.AgeDays, cfg.HalfLifeDays)
 		artistW[p.ArtistID] += e
@@ -154,15 +174,16 @@ func accumulate(
 			tagW[tag] += e
 		}
 		foldEnrichedTags(tagW, playedTags[p.TrackID], e, cfg.EnrichedTagScale)
+		foldEra(eraW, p.ReleaseDate, e, cfg.EraScale)
 	}
 
 	likedTracks, err := q.ListLikedTrackTasteInputsForUser(ctx, userID)
 	if err != nil {
-		return nil, nil, fmt.Errorf("taste: load liked tracks: %w", err)
+		return nil, nil, nil, fmt.Errorf("taste: load liked tracks: %w", err)
 	}
 	likedTagRows, err := q.ListLikedTrackTagsForUser(ctx, userID)
 	if err != nil {
-		return nil, nil, fmt.Errorf("taste: load liked track tags: %w", err)
+		return nil, nil, nil, fmt.Errorf("taste: load liked track tags: %w", err)
 	}
 	likedTags := groupTagsByTrack(likedTagRows)
 	for _, lt := range likedTracks {
@@ -171,16 +192,17 @@ func accumulate(
 			tagW[tag] += cfg.TagLikeBonus
 		}
 		foldEnrichedTags(tagW, likedTags[lt.TrackID], cfg.TagLikeBonus, cfg.EnrichedTagScale)
+		foldEra(eraW, lt.ReleaseDate, cfg.TagLikeBonus, cfg.EraScale)
 	}
 
 	likedArtists, err := q.ListLikedArtistIDsForUser(ctx, userID)
 	if err != nil {
-		return nil, nil, fmt.Errorf("taste: load liked artists: %w", err)
+		return nil, nil, nil, fmt.Errorf("taste: load liked artists: %w", err)
 	}
 	for _, aid := range likedArtists {
 		artistW[aid] += cfg.ArtistLikeBonus
 	}
-	return artistW, tagW, nil
+	return artistW, tagW, eraW, nil
 }
 
 // groupTagsByTrack buckets flat (track, tag, weight) rows by track id so
@@ -206,10 +228,33 @@ func foldEnrichedTags(tagW map[string]float64, tags []dbq.TrackTag, base, scale
 	}
 }
 
+// foldEra adds a track's decade bucket to eraW weighted by base × scale — base
+// is the play's decayed engagement or the tag-like bonus, scale (EraScale)
+// dials the facet's strength. scale=0 disables the era facet; undated tracks
+// (decade "") contribute nothing so they don't pollute the profile.
+func foldEra(eraW map[string]float64, d pgtype.Date, base, scale float64) {
+	if scale == 0 {
+		return
+	}
+	if decade := decadeOf(d); decade != "" {
+		eraW[decade] += base * scale
+	}
+}
+
+// decadeOf maps an album release date to a decade bucket like "1990s", or ""
+// for an absent/invalid date. Mirrored by the recommendation scorer so a
+// candidate's era is derived the same way it was learned.
+func decadeOf(d pgtype.Date) string {
+	if !d.Valid {
+		return ""
+	}
+	return fmt.Sprintf("%ds", (d.Time.Year()/10)*10)
+}
+
 // persist atomic-replaces the user's profile rows inside one transaction.
 func persist(
 	ctx context.Context, pool *pgxpool.Pool, userID pgtype.UUID,
-	artists []weighted[pgtype.UUID], tags []weighted[string],
+	artists []weighted[pgtype.UUID], tags, eras []weighted[string],
 ) error {
 	tx, err := pool.Begin(ctx)
 	if err != nil {
@@ -238,6 +283,16 @@ func persist(
 			return fmt.Errorf("taste: insert tag: %w", err)
 		}
 	}
+	if err := qtx.DeleteTasteProfileErasForUser(ctx, userID); err != nil {
+		return fmt.Errorf("taste: delete eras: %w", err)
+	}
+	for _, e := range eras {
+		if err := qtx.InsertTasteProfileEra(ctx, dbq.InsertTasteProfileEraParams{
+			UserID: userID, Era: e.Key, Weight: e.Weight,
+		}); err != nil {
+			return fmt.Errorf("taste: insert era: %w", err)
+		}
+	}
 	if err := tx.Commit(ctx); err != nil {
 		return fmt.Errorf("taste: commit: %w", err)
 	}