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_test.go

@@ -4,10 +4,39 @@ import (
 	"testing"
 
 	"github.com/jackc/pgx/v5/pgtype"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 )
 
 func strLess(a, b string) bool { return a < b }
 
+func TestFoldEnrichedTags(t *testing.T) {
+	tagW := map[string]float64{"rock": 1.0} // pre-existing ID3 genre weight
+	tags := []dbq.TrackTag{
+		{Tag: "shoegaze", Weight: 1.0},
+		{Tag: "melancholic", Weight: 0.5},
+		{Tag: "rock", Weight: 0.4}, // overlaps genre → accumulates
+	}
+	// base=2 (engagement), scale=0.5 → contribution = 2 * weight * 0.5 = weight.
+	foldEnrichedTags(tagW, tags, 2.0, 0.5)
+	if got := tagW["shoegaze"]; got != 1.0 {
+		t.Errorf("shoegaze = %v, want 1.0", got)
+	}
+	if got := tagW["melancholic"]; got != 0.5 {
+		t.Errorf("melancholic = %v, want 0.5", got)
+	}
+	if got := tagW["rock"]; got != 1.4 { // 1.0 genre + 2*0.4*0.5
+		t.Errorf("rock = %v, want 1.4", got)
+	}
+
+	// scale=0 disables the enriched contribution entirely.
+	base := map[string]float64{"rock": 1.0}
+	foldEnrichedTags(base, tags, 2.0, 0)
+	if len(base) != 1 || base["rock"] != 1.0 {
+		t.Errorf("scale=0 should leave tagW untouched; got %v", base)
+	}
+}
+
 func TestRankWeighted_DropsEpsilonAndSortsDesc(t *testing.T) {
 	m := map[string]float64{"a": 3.0, "b": 1.0, "tiny": 0.01, "d": -2.0}
 	got := rankWeighted(m, 0.05, 10, 10, strLess)