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.go

@@ -0,0 +1,58 @@
+// Package mood maps folksonomy tags (from the track_tags cache, #1490) to a
+// small set of canonical mood buckets (#1534, milestone #160 Opt 2b). Both the
+// taste-profile builder and the recommendation scorer classify a track's tags
+// through Of() so a track's mood is derived the same way on both sides.
+//
+// The vocabulary is intentionally narrow: only words that clearly denote mood
+// (not genre). A track with no mood-word tags contributes nothing — the mood
+// facet is a supplement whose coverage grows with folksonomy enrichment
+// (thin on MusicBrainz-only, richer once Last.fm is keyed).
+package mood
+
+import "strings"
+
+// vocab maps a lowercased folksonomy tag to its canonical mood bucket. Several
+// synonyms collapse to one bucket so "sad" / "melancholy" / "melancholic" all
+// score the same dimension.
+var vocab = map[string]string{
+	// melancholic
+	"melancholic": "melancholic", "melancholy": "melancholic", "sad": "melancholic",
+	"sombre": "melancholic", "somber": "melancholic", "wistful": "melancholic",
+	"bittersweet": "melancholic", "mournful": "melancholic",
+	// energetic
+	"energetic": "energetic", "energy": "energetic", "upbeat": "energetic",
+	"uplifting": "energetic", "party": "energetic", "anthemic": "energetic",
+	// chill
+	"chill": "chill", "chillout": "chill", "chilled": "chill", "mellow": "chill",
+	"relaxing": "chill", "relaxed": "chill", "calm": "chill", "laid-back": "chill",
+	"laidback": "chill", "soothing": "chill",
+	// aggressive
+	"aggressive": "aggressive", "angry": "aggressive", "intense": "aggressive",
+	"heavy": "aggressive", "brutal": "aggressive",
+	// dark
+	"dark": "dark", "moody": "dark", "brooding": "dark", "haunting": "dark", "eerie": "dark",
+	// dreamy
+	"dreamy": "dreamy", "ethereal": "dreamy", "atmospheric": "dreamy",
+	"ambient": "dreamy", "hypnotic": "dreamy",
+	// happy
+	"happy": "happy", "cheerful": "happy", "feel good": "happy", "feel-good": "happy",
+	"feelgood": "happy", "joyful": "happy", "fun": "happy",
+	// romantic
+	"romantic": "romantic", "sensual": "romantic", "sexy": "romantic", "sentimental": "romantic",
+}
+
+// Of returns the distinct canonical mood buckets present in tags, lowercasing +
+// trimming each tag before lookup. Returns nil when no tag maps to a mood, so
+// callers get no mood signal for a track with only genre/descriptive tags.
+func Of(tags []string) []string {
+	var out []string
+	seen := make(map[string]bool)
+	for _, t := range tags {
+		key := strings.ToLower(strings.TrimSpace(t))
+		if m, ok := vocab[key]; ok && !seen[m] {
+			seen[m] = true
+			out = append(out, m)
+		}
+	}
+	return out
+}