feat(taste): fold enriched folksonomy tags into the profile (#1490 Step 3)

The taste recompute's tag facet now unions the cached track_tags
(MusicBrainz/Last.fm folksonomy tags) alongside raw ID3 genre, so a coarse
"Rock" gains "post-punk / shoegaze / melancholic".

- taste_profile.sql: ListPlayEngagementInputsForUser +
  ListLikedTrackTasteInputsForUser now return track_id to key the
  enriched-tag lookup.
- accumulate(): for each play, fold its track's enriched tags weighted by
  engagement × tag.weight × EnrichedTagScale; for each liked track, by the
  tag-like bonus × tag.weight × scale. A track with no cached tags
  contributes genre only (graceful).
- New Config.EnrichedTagScale (default 0.5) — enriched tags augment the
  ID3 signal without swamping it; 0 = genre-only. Flows through
  recsettings.TasteConfig() (starts from DefaultConfig). Promoting it into
  the admin tuning lab is a small follow-up.

Unit-tested the pure foldEnrichedTags helper (overlap accumulation +
scale=0 disable).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/taste/profile.go

@@ -36,6 +36,16 @@ type Config struct {
 	TrackLikeBonus  float64
 	TagLikeBonus    float64
 
+	// EnrichedTagScale weights folksonomy tags (from the track_tags cache,
+	// #1490) relative to a track's raw ID3 genre when both fold into the tag
+	// facet. Each enriched tag contributes base × tag.weight × this scale,
+	// where base is the play's decayed engagement (or the tag-like bonus for
+	// a liked track) and tag.weight is the normalized folksonomy strength in
+	// [0,1]. Below 1 so the richer-but-noisier enriched vocabulary augments
+	// the ID3 genre signal without swamping it. 0 disables the enriched
+	// contribution entirely (falls back to genre-only).
+	EnrichedTagScale 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
@@ -58,19 +68,20 @@ type Config struct {
 // DefaultConfig returns the tuned starting configuration.
 func DefaultConfig() Config {
 	return Config{
-		Engagement:      DefaultEngagementParams(),
-		HalfLifeDays:    75,
-		WindowDays:      270,
-		ArtistLikeBonus: 3.0,
-		TrackLikeBonus:  1.0,
-		TagLikeBonus:    0.5,
-		ArtistFloor:     -3.0,
-		TagFloor:        -3.0,
-		WeightEpsilon:   0.05,
-		MaxArtists:      500,
-		MaxNegArtists:   150,
-		MaxTags:         200,
-		MaxNegTags:      60,
+		Engagement:       DefaultEngagementParams(),
+		HalfLifeDays:     75,
+		WindowDays:       270,
+		ArtistLikeBonus:  3.0,
+		TrackLikeBonus:   1.0,
+		TagLikeBonus:     0.5,
+		EnrichedTagScale: 0.5,
+		ArtistFloor:      -3.0,
+		TagFloor:         -3.0,
+		WeightEpsilon:    0.05,
+		MaxArtists:       500,
+		MaxNegArtists:    150,
+		MaxTags:          200,
+		MaxNegTags:       60,
 	}
 }
 
@@ -121,6 +132,19 @@ func accumulate(
 	if err != nil {
 		return 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
+	// "post-punk / shoegaze / melancholic" vocabulary. Window matches the
+	// play window; a track with no cached tags simply contributes genre only.
+	playedTagRows, err := q.ListPlayedTrackTagsForUser(ctx, dbq.ListPlayedTrackTagsForUserParams{
+		UserID:  userID,
+		Column2: int32(cfg.WindowDays),
+	})
+	if err != nil {
+		return 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)
 	for _, p := range plays {
@@ -129,17 +153,24 @@ func accumulate(
 		for _, tag := range splitGenres(p.Genre) {
 			tagW[tag] += e
 		}
+		foldEnrichedTags(tagW, playedTags[p.TrackID], e, cfg.EnrichedTagScale)
 	}
 
 	likedTracks, err := q.ListLikedTrackTasteInputsForUser(ctx, userID)
 	if err != nil {
 		return 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)
+	}
+	likedTags := groupTagsByTrack(likedTagRows)
 	for _, lt := range likedTracks {
 		artistW[lt.ArtistID] += cfg.TrackLikeBonus
 		for _, tag := range splitGenres(lt.Genre) {
 			tagW[tag] += cfg.TagLikeBonus
 		}
+		foldEnrichedTags(tagW, likedTags[lt.TrackID], cfg.TagLikeBonus, cfg.EnrichedTagScale)
 	}
 
 	likedArtists, err := q.ListLikedArtistIDsForUser(ctx, userID)
@@ -152,6 +183,29 @@ func accumulate(
 	return artistW, tagW, nil
 }
 
+// groupTagsByTrack buckets flat (track, tag, weight) rows by track id so
+// each play/like can fold in its track's enriched tags in one lookup.
+func groupTagsByTrack(rows []dbq.TrackTag) map[pgtype.UUID][]dbq.TrackTag {
+	m := make(map[pgtype.UUID][]dbq.TrackTag)
+	for _, r := range rows {
+		m[r.TrackID] = append(m[r.TrackID], r)
+	}
+	return m
+}
+
+// foldEnrichedTags adds each enriched tag to tagW weighted by
+// base × tag.weight × scale — base is the play's decayed engagement or the
+// tag-like bonus, tag.weight is the folksonomy strength in [0,1]. scale=0
+// disables the enriched contribution (genre-only fallback).
+func foldEnrichedTags(tagW map[string]float64, tags []dbq.TrackTag, base, scale float64) {
+	if scale == 0 {
+		return
+	}
+	for _, t := range tags {
+		tagW[t.Tag] += base * t.Weight * scale
+	}
+}
+
 // persist atomic-replaces the user's profile rows inside one transaction.
 func persist(
 	ctx context.Context, pool *pgxpool.Pool, userID pgtype.UUID,