feat(taste): mood taste facet — #1534

Milestone #160 Opt 2b (mood half of the era+mood option). A fourth taste
facet alongside artists + genre tags + eras: signed weights over canonical
mood buckets (melancholic / energetic / chill / …) derived from a track's
enriched folksonomy tags (#1490).

- internal/mood: shared vocabulary — Of(tags) maps folksonomy tags to
  canonical mood buckets (synonyms collapse). Imported by both the taste
  builder and the scorer so a track's mood is derived identically.
- Migration 0047: taste_profile_moods table + taste_tuning.mood_scale
  (DEFAULT 0.5).
- Build side (internal/taste): Config.MoodScale ([0,1] damper, mirrors
  EraScale); accumulate folds each play/like's mood buckets at
  base*MoodScale; persist atomic-replaces the mood rows.
- Scorer (internal/recommendation): TasteProfile gains a mood term
  (own tanh scale + additive 0.12 share, so it never weakens the existing
  signal when a track has no mood tags). Match now takes the candidate's
  mood buckets; loaded per candidate (ListTrackTagsForTracks → mood.Of) in
  the primary similarity loader only — the near-whole-library fallback
  pool passes nil (mood → 0) to avoid a full-library tag scan.
- Tuning lab: mood_scale threaded through recsettings + admin API + web
  card ("Mood weight" row) + Go/web tests.

Coverage is partial (grows with tag enrichment; richer once Last.fm is
keyed), so mood is a supplement — neutral for tracks with no mood tags.

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

internal/mood/mood_test.go

@@ -0,0 +1,43 @@
+package mood
+
+import (
+	"sort"
+	"testing"
+)
+
+func TestOf(t *testing.T) {
+	cases := []struct {
+		name string
+		in   []string
+		want []string
+	}{
+		{"synonyms collapse", []string{"sad", "melancholy", "melancholic"}, []string{"melancholic"}},
+		{"case + whitespace", []string{" CHILL ", "Mellow"}, []string{"chill"}},
+		{"multiple buckets", []string{"energetic", "dark"}, []string{"dark", "energetic"}},
+		{"genre-only → none", []string{"rock", "shoegaze", "post-punk"}, nil},
+		{"empty", nil, nil},
+	}
+	for _, c := range cases {
+		got := Of(c.in)
+		sort.Strings(got)
+		want := append([]string(nil), c.want...)
+		sort.Strings(want)
+		if len(got) != len(want) {
+			t.Errorf("%s: Of(%v) = %v, want %v", c.name, c.in, got, want)
+			continue
+		}
+		for i := range got {
+			if got[i] != want[i] {
+				t.Errorf("%s: Of(%v) = %v, want %v", c.name, c.in, got, want)
+				break
+			}
+		}
+	}
+}
+
+func TestOf_Dedupes(t *testing.T) {
+	// Three tags mapping to the same bucket yield one entry.
+	if got := Of([]string{"chill", "mellow", "relaxed"}); len(got) != 1 || got[0] != "chill" {
+		t.Errorf("Of dedupe = %v, want [chill]", got)
+	}
+}